[GRASS-dev] Write array to (point) attribute table

GRASS GIS GRASS GIS developer
1

This must be a very basic question, but I can’t find an easy/direct way to do this. In python, if I have an array with values with a length equal to the number of rows in an attribute table of a (point) vector layer, how can I write those values to a new column in that attribute table. I can of course first create the column, but than how to update that column with the values in the array?

Cheers,

Paulo

2

On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel <p.vanbreugel@gmail.com>
wrote:

This must be a very basic question, but I can't find an easy/direct way to
do this. In python, if I have an array with values with a length equal to
the number of rows in an attribute table of a (point) vector layer, how can
I write those values to a new column in that attribute table. I can of
course first create the column, but than how to update that column with the
values in the array?

it should be pretty easy to do with pygrass, unfortunately there is no
example on assigning attributes in the official documentation [1], but it
should be pretty easy, something like that (not tested):

with VectorTopo('myvector', mode='w') as vectormap:
    for feature in vectormap:
        feature.attrs['mycolumn'] = value

Anna

[1]
https://grass.osgeo.org/grass70/manuals/libpython/pygrass_vector.html#working-with-vector-objects

Cheers,

Paulo

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grass-dev mailing list
grass-dev@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-dev

3

On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com>
wrote:

On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel <p.vanbreugel@gmail.com
> wrote:

This must be a very basic question, but I can't find an easy/direct way
to do this. In python, if I have an array with values with a length equal
to the number of rows in an attribute table of a (point) vector layer, how
can I write those values to a new column in that attribute table. I can of
course first create the column, but than how to update that column with the
values in the array?

it should be pretty easy to do with pygrass, unfortunately there is no
example on assigning attributes in the official documentation [1], but it
should be pretty easy, something like that (not tested):

with VectorTopo('myvector', mode='w') as vectormap:
    for feature in vectormap:
        feature.attrs['mycolumn'] = value

Thanks, but that seems to write the vector back without attribute table

Anna

[1]
https://grass.osgeo.org/grass70/manuals/libpython/pygrass_vector.html#working-with-vector-objects

Cheers,

Paulo

_______________________________________________
grass-dev mailing list
grass-dev@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-dev

4

On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com>
wrote:

On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

This must be a very basic question, but I can't find an easy/direct way
to do this. In python, if I have an array with values with a length equal to
the number of rows in an attribute table of a (point) vector layer, how can
I write those values to a new column in that attribute table. I can of
course first create the column, but than how to update that column with the
values in the array?

it should be pretty easy to do with pygrass, unfortunately there is no
example on assigning attributes in the official documentation [1], but it
should be pretty easy, something like that (not tested):

with VectorTopo('myvector', mode='w') as vectormap:
    for feature in vectormap:
        feature.attrs['mycolumn'] = value

Thanks, but that seems to write the vector back without attribute table

You have to save the changes in the database out from your cycle, with:

vectormap.table.conn.commit()

Pietro

5

On Wed, Sep 30, 2015 at 11:53 AM, Pietro <peter.zamb@gmail.com> wrote:

On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:
>
>
> On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com>
> wrote:
>>
>>
>>
>> On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
>> <p.vanbreugel@gmail.com> wrote:
>>>
>>> This must be a very basic question, but I can't find an easy/direct way
>>> to do this. In python, if I have an array with values with a length
equal to
>>> the number of rows in an attribute table of a (point) vector layer,
how can
>>> I write those values to a new column in that attribute table. I can of
>>> course first create the column, but than how to update that column
with the
>>> values in the array?
>>
>>
>> it should be pretty easy to do with pygrass, unfortunately there is no
>> example on assigning attributes in the official documentation [1], but
it
>> should be pretty easy, something like that (not tested):
>>
>> with VectorTopo('myvector', mode='w') as vectormap:
>> for feature in vectormap:
>> feature.attrs['mycolumn'] = value
>>
>>
> Thanks, but that seems to write the vector back without attribute table

You have to save the changes in the database out from your cycle, with:

vectormap.table.conn.commit()

Thanks Pietro. I am, however, not sure I understand (I tried to use it, but
thanks to my limited experience in Python / pygrass not much luck). Just to
be more specific, I am trying to create a script that divides points in
training and test groups, similar to v.kcv, but with points clustered in
space. E.g.,

# Create vector
grass.run_command("v.random", output="testB", npoints=10, overwrite=True)
grass.run_command("v.db.addtable", map="testB", columns="X DOUBLE
PRECISION,Y DOUBLE PRECISION,GR INTEGER")
grass.run_command("v.to.db", map="test", option="coor", columns="X,Y")

# Create groups
vectmap = 'test'
cvals = array(grass.vector_db_select(vectmap, layer = int(1), columns =
'X,Y')['values'].values()).astype(np.float)
centroids,_ = kmeans(cvals,2)
idx,_ = vq(cvals,centroids)

# write results to tabel
Now I would like to write idx to the column 'GR' in the attribute table of
'test'.

p.s. I am first creating the XY columns now, but is there a function to get
the coordinates (cvals) in pygrass directly?

Pietro

6

On 30/09/15 15:27, Paulo van Breugel wrote:

On Wed, Sep 30, 2015 at 11:53 AM, Pietro <peter.zamb@gmail.com
<mailto:peter.zamb@gmail.com>> wrote:

    On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
    <p.vanbreugel@gmail.com <mailto:p.vanbreugel@gmail.com>> wrote:
    >
    > On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com <mailto:kratochanna@gmail.com>>
    > wrote:
    >>
    >> On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
    >> <p.vanbreugel@gmail.com <mailto:p.vanbreugel@gmail.com>> wrote:
    >>>
    >>> This must be a very basic question, but I can't find an easy/direct way
    >>> to do this. In python, if I have an array with values with a length equal to
    >>> the number of rows in an attribute table of a (point) vector layer, how can
    >>> I write those values to a new column in that attribute table. I can of
    >>> course first create the column, but than how to update that column with the
    >>> values in the array?
    >>
    >> it should be pretty easy to do with pygrass, unfortunately there is no
    >> example on assigning attributes in the official documentation [1], but it
    >> should be pretty easy, something like that (not tested):
    >>
    >> with VectorTopo('myvector', mode='w') as vectormap:
    >> for feature in vectormap:
    >> feature.attrs['mycolumn'] = value
    >>
    > Thanks, but that seems to write the vector back without attribute table

    You have to save the changes in the database out from your cycle, with:

    vectormap.table.conn.commit()

Thanks Pietro. I am, however, not sure I understand (I tried to use it,
but thanks to my limited experience in Python / pygrass not much luck).
Just to be more specific, I am trying to create a script that divides
points in training and test groups, similar to v.kcv, but with points
clustered in space. E.g.,

# Create vector
grass.run_command("v.random", output="testB", npoints=10, overwrite=True)
grass.run_command("v.db.addtable", map="testB", columns="X DOUBLE
PRECISION,Y DOUBLE PRECISION,GR INTEGER")
grass.run_command("v.to.db", map="test", option="coor", columns="X,Y")

# Create groups
vectmap = 'test'
cvals = array(grass.vector_db_select(vectmap, layer = int(1), columns =
'X,Y')['values'].values()).astype(np.float)
centroids,_ = kmeans(cvals,2)
idx,_ = vq(cvals,centroids)

# write results to tabel
Now I would like to write idx to the column 'GR' in the attribute table
of 'test'.

p.s. I am first creating the XY columns now, but is there a function to
get the coordinates (cvals) in pygrass directly?

Pygrass has other options, but you can always call v.to.db with the -p flag and read the output directly. See one example at [1].

Moritz

[1] https://trac.osgeo.org/grass/browser/grass-addons/grass7/vector/v.centerline/v.centerline.py#L198

7

On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel <p.vanbreugel@gmail.com>
wrote:

On Wed, Sep 30, 2015 at 11:53 AM, Pietro <peter.zamb@gmail.com> wrote:

On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:
>
>
> On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com>
> wrote:
>>
>>
>>
>> On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
>> <p.vanbreugel@gmail.com> wrote:
>>>
>>> This must be a very basic question, but I can't find an easy/direct
way
>>> to do this. In python, if I have an array with values with a length
equal to
>>> the number of rows in an attribute table of a (point) vector layer,
how can
>>> I write those values to a new column in that attribute table. I can of
>>> course first create the column, but than how to update that column
with the
>>> values in the array?
>>
>>
>> it should be pretty easy to do with pygrass, unfortunately there is no
>> example on assigning attributes in the official documentation [1],
but it
>> should be pretty easy, something like that (not tested):
>>
>> with VectorTopo('myvector', mode='w') as vectormap:
>> for feature in vectormap:
>> feature.attrs['mycolumn'] = value
>>
>>
> Thanks, but that seems to write the vector back without attribute table

You have to save the changes in the database out from your cycle, with:

vectormap.table.conn.commit()

Thanks Pietro. I am, however, not sure I understand (I tried to use it,
but thanks to my limited experience in Python / pygrass not much luck).
Just to be more specific, I am trying to create a script that divides
points in training and test groups, similar to v.kcv, but with points
clustered in space. E.g.,

# Create vector
grass.run_command("v.random", output="testB", npoints=10, overwrite=True)
grass.run_command("v.db.addtable", map="testB", columns="X DOUBLE
PRECISION,Y DOUBLE PRECISION,GR INTEGER")
grass.run_command("v.to.db", map="test", option="coor", columns="X,Y")

# Create groups
vectmap = 'test'
cvals = array(grass.vector_db_select(vectmap, layer = int(1), columns =
'X,Y')['values'].values()).astype(np.float)
centroids,_ = kmeans(cvals,2)
idx,_ = vq(cvals,centroids)

# write results to tabel
Now I would like to write idx to the column 'GR' in the attribute table of
'test'.

p.s. I am first creating the XY columns now, but is there a function to
get the coordinates (cvals) in pygrass directly?

I don't fully understand the example, but yes, you can get coordinates:

with VectorTopo('myvector', mode='w') as vectormap:
    for feature in vectormap:
        print feature.x
        print feature.y

Pietro

8

Thanks for the quick response. I basically have a list with values (idx in the example above) which I like to add as a column to the attribute table of an existing vector (point layer). The length of idx is equal to the number of rows in the attribute table. The solution of Anna seems like an elegant solution (and easier and more flexible than other solutions I tried using e.g., sqlite3). However, as I wrote, I end up with a vector without attribute table. You wrote that I “have to save the changes in the database out from your cycle, with: vectormap.table.conn.commit()”. I am, however, not sure what you mean with ‘out from your cycle’ or how to implement that. Great, thanks. I did not find this in the manual. If it is not there, perhaps it would be something worth including? I would not mind providing a text, but I am not sure what would be the best way to do that.

···

On 30-09-15 16:18, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel <p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 11:53 AM, Pietro <peter.zamb@gmail.com> wrote:

On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com>
wrote:

On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

This must be a very basic question, but I can’t find an easy/direct way
to do this. In python, if I have an array with values with a length equal to
the number of rows in an attribute table of a (point) vector layer, how can
I write those values to a new column in that attribute table. I can of
course first create the column, but than how to update that column with the
values in the array?

it should be pretty easy to do with pygrass, unfortunately there is no
example on assigning attributes in the official documentation [1], but it
should be pretty easy, something like that (not tested):

with VectorTopo(‘myvector’, mode=‘w’) as vectormap:
for feature in vectormap:
feature.attrs[‘mycolumn’] = value

Thanks, but that seems to write the vector back without attribute table

You have to save the changes in the database out from your cycle, with:

vectormap.table.conn.commit()

Thanks Pietro. I am, however, not sure I understand (I tried to use it, but thanks to my limited experience in Python / pygrass not much luck). Just to be more specific, I am trying to create a script that divides points in training and test groups, similar to v.kcv, but with points clustered in space. E.g.,

Create vector

grass.run_command(“v.random”, output=“testB”, npoints=10, overwrite=True)
grass.run_command(“v.db.addtable”, map=“testB”, columns=“X DOUBLE PRECISION,Y DOUBLE PRECISION,GR INTEGER”)
grass.run_command(“v.to.db”, map=“test”, option=“coor”, columns=“X,Y”)

Create groups

vectmap = ‘test’
cvals = array(grass.vector_db_select(vectmap, layer = int(1), columns = ‘X,Y’)[‘values’].values()).astype(np.float)
centroids,_ = kmeans(cvals,2)
idx,_ = vq(cvals,centroids)

write results to tabel

Now I would like to write idx to the column ‘GR’ in the attribute table of ‘test’.

p.s. I am first creating the XY columns now, but is there a function to get the coordinates (cvals) in pygrass directly?

I don’t fully understand the example,

but yes, you can get coordinates:

with VectorTopo(‘myvector’, mode=‘w’) as vectormap:
for feature in vectormap:
print feature.x

print feature.y

Pietro

9

On Wed, Sep 30, 2015 at 11:20 AM, Paulo van Breugel <p.vanbreugel@gmail.com>
wrote:

On 30-09-15 16:18, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel <
<p.vanbreugel@gmail.com>p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 11:53 AM, Pietro < <peter.zamb@gmail.com>
peter.zamb@gmail.com> wrote:

On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:
>
>
> On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com
>
> wrote:
>>
>>
>>
>> On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
>> <p.vanbreugel@gmail.com> wrote:
>>>
>>> This must be a very basic question, but I can't find an easy/direct
way
>>> to do this. In python, if I have an array with values with a length
equal to
>>> the number of rows in an attribute table of a (point) vector layer,
how can
>>> I write those values to a new column in that attribute table. I can
of
>>> course first create the column, but than how to update that column
with the
>>> values in the array?
>>
>>
>> it should be pretty easy to do with pygrass, unfortunately there is no
>> example on assigning attributes in the official documentation [1],
but it
>> should be pretty easy, something like that (not tested):
>>
>> with VectorTopo('myvector', mode='w') as vectormap:
>> for feature in vectormap:
>> feature.attrs['mycolumn'] = value
>>
>>
> Thanks, but that seems to write the vector back without attribute table

You have to save the changes in the database out from your cycle, with:

vectormap.table.conn.commit()

Thanks Pietro. I am, however, not sure I understand (I tried to use it,
but thanks to my limited experience in Python / pygrass not much luck).
Just to be more specific, I am trying to create a script that divides
points in training and test groups, similar to v.kcv, but with points
clustered in space. E.g.,

# Create vector
grass.run_command("v.random", output="testB", npoints=10, overwrite=True)
grass.run_command("v.db.addtable", map="testB", columns="X DOUBLE
PRECISION,Y DOUBLE PRECISION,GR INTEGER")
grass.run_command("v.to.db", map="test", option="coor", columns="X,Y")

# Create groups
vectmap = 'test'
cvals = array(grass.vector_db_select(vectmap, layer = int(1), columns =
'X,Y')['values'].values()).astype(np.float)
centroids,_ = kmeans(cvals,2)
idx,_ = vq(cvals,centroids)

# write results to tabel
Now I would like to write idx to the column 'GR' in the attribute table
of 'test'.

p.s. I am first creating the XY columns now, but is there a function to
get the coordinates (cvals) in pygrass directly?

I don't fully understand the example,

Thanks for the quick response. I basically have a list with values (idx in
the example above) which I like to add as a column to the attribute table
of an existing vector (point layer). The length of idx is equal to the
number of rows in the attribute table. The solution of Anna seems like an
elegant solution (and easier and more flexible than other solutions I tried
using e.g., sqlite3). However, as I wrote, I end up with a vector without
attribute table. You wrote that I "have to save the changes in the database
out from your cycle, with: vectormap.table.conn.commit()". I am, however,
not sure what you mean with 'out from your cycle' or how to implement that.

probably after the for cycle ends you would call this
'vectormap.table.conn.commit()', if it doesn't works, try to put it in the
cycle (I am not sure what is supposed to work).

but yes, you can get coordinates:

with VectorTopo('myvector', mode='w') as vectormap:
    for feature in vectormap:
        print feature.x
        print feature.y

Great, thanks. I did not find this in the manual. If it is not there,
perhaps it would be something worth including? I would not mind providing a
text, but I am not sure what would be the best way to do that.

Technically, it's in the manual:
https://grass.osgeo.org/grass70/manuals/libpython/pygrass_vector.html#geometry-classes

but we are lacking more examples of often used constructions. Some other
examples are available in the recent workshop we did:

https://github.com/wenzeslaus/python-grass-addon/blob/master/02_pygrass_library.ipynb

Pietro

10

I did try both, with no luck so-far. I’ll give it another try though, perhaps I did something else wrong. Thanks, I will have a look at it.

···

On 30-09-15 17:47, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 11:20 AM, Paulo van Breugel <p.vanbreugel@gmail.com> wrote:

On 30-09-15 16:18, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel <p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 11:53 AM, Pietro <peter.zamb@gmail.com> wrote:

On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com>
wrote:

On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

This must be a very basic question, but I can’t find an easy/direct way
to do this. In python, if I have an array with values with a length equal to
the number of rows in an attribute table of a (point) vector layer, how can
I write those values to a new column in that attribute table. I can of
course first create the column, but than how to update that column with the
values in the array?

it should be pretty easy to do with pygrass, unfortunately there is no
example on assigning attributes in the official documentation [1], but it
should be pretty easy, something like that (not tested):

with VectorTopo(‘myvector’, mode=‘w’) as vectormap:
for feature in vectormap:
feature.attrs[‘mycolumn’] = value

Thanks, but that seems to write the vector back without attribute table

You have to save the changes in the database out from your cycle, with:

vectormap.table.conn.commit()

Thanks Pietro. I am, however, not sure I understand (I tried to use it, but thanks to my limited experience in Python / pygrass not much luck). Just to be more specific, I am trying to create a script that divides points in training and test groups, similar to v.kcv, but with points clustered in space. E.g.,

Create vector

grass.run_command(“v.random”, output=“testB”, npoints=10, overwrite=True)
grass.run_command(“v.db.addtable”, map=“testB”, columns=“X DOUBLE PRECISION,Y DOUBLE PRECISION,GR INTEGER”)
grass.run_command(“v.to.db”, map=“test”, option=“coor”, columns=“X,Y”)

Create groups

vectmap = ‘test’
cvals = array(grass.vector_db_select(vectmap, layer = int(1), columns = ‘X,Y’)[‘values’].values()).astype(np.float)
centroids,_ = kmeans(cvals,2)
idx,_ = vq(cvals,centroids)

write results to tabel

Now I would like to write idx to the column ‘GR’ in the attribute table of ‘test’.

p.s. I am first creating the XY columns now, but is there a function to get the coordinates (cvals) in pygrass directly?

I don’t fully understand the example,

Thanks for the quick response. I basically have a list with values (idx in the example above) which I like to add as a column to the attribute table of an existing vector (point layer). The length of idx is equal to the number of rows in the attribute table. The solution of Anna seems like an elegant solution (and easier and more flexible than other solutions I tried using e.g., sqlite3). However, as I wrote, I end up with a vector without attribute table. You wrote that I “have to save the changes in the database out from your cycle, with: vectormap.table.conn.commit()”. I am, however, not sure what you mean with ‘out from your cycle’ or how to implement that.

probably after the for cycle ends you would call this ‘vectormap.table.conn.commit()’, if it doesn’t works, try to put it in the cycle (I am not sure what is supposed to work).

but yes, you can get coordinates:

with VectorTopo(‘myvector’, mode=‘w’) as vectormap:
for feature in vectormap:
print feature.x

print feature.y

Great, thanks. I did not find this in the manual. If it is not there, perhaps it would be something worth including? I would not mind providing a text, but I am not sure what would be the best way to do that.

Technically, it’s in the manual:
https://grass.osgeo.org/grass70/manuals/libpython/pygrass_vector.html#geometry-classes

but we are lacking more examples of often used constructions. Some other examples are available in the recent workshop we did:

https://github.com/wenzeslaus/python-grass-addon/blob/master/02_pygrass_library.ipynb

Pietro

11

Tried out again, but after running the code below, I end up with an empty vector layer (no points and no attribute table). Same if I put the vectormap.table.conn.commit() in the for cycle.

···

On 30-09-15 17:52, Paulo van Breugel wrote:

I did try both, with no luck so-far. I’ll give it another try though, perhaps I did something else wrong.

import grass.script as grass

from grass.pygrass.vector import VectorTopo

grass.run_command("v.random", output="testB", npoints=10, overwrite=True)

grass.run_command("v.db.addtable", map="testB", columns="GR INTEGER")

with VectorTopo('testB', mode='w') as vectormap:

    for feature in vectormap:

        feature.attrs['GR'] = 9

        vectormap.table.conn.commit()

but yes, you can get coordinates:

with VectorTopo(‘myvector’, mode=‘w’) as vectormap:
for feature in vectormap:
print feature.x

print feature.y

Great, thanks. I did not find this in the manual. If it is not there, perhaps it would be something worth including? I would not mind providing a text, but I am not sure what would be the best way to do that.

Technically, it’s in the manual:
https://grass.osgeo.org/grass70/manuals/libpython/pygrass_vector.html#geometry-classes

but we are lacking more examples of often used constructions. Some other examples are available in the recent workshop we did:

https://github.com/wenzeslaus/python-grass-addon/blob/master/02_pygrass_library.ipynb

Thanks, I will have a look at it.

Pietro

On 30-09-15 17:47, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 11:20 AM, Paulo van Breugel <p.vanbreugel@gmail.com> wrote:

On 30-09-15 16:18, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel <p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 11:53 AM, Pietro <peter.zamb@gmail.com> wrote:

On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com>
wrote:

On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

This must be a very basic question, but I can’t find an easy/direct way
to do this. In python, if I have an array with values with a length equal to
the number of rows in an attribute table of a (point) vector layer, how can
I write those values to a new column in that attribute table. I can of
course first create the column, but than how to update that column with the
values in the array?

it should be pretty easy to do with pygrass, unfortunately there is no
example on assigning attributes in the official documentation [1], but it
should be pretty easy, something like that (not tested):

with VectorTopo(‘myvector’, mode=‘w’) as vectormap:
for feature in vectormap:
feature.attrs[‘mycolumn’] = value

Thanks, but that seems to write the vector back without attribute table

You have to save the changes in the database out from your cycle, with:

vectormap.table.conn.commit()

Thanks Pietro. I am, however, not sure I understand (I tried to use it, but thanks to my limited experience in Python / pygrass not much luck). Just to be more specific, I am trying to create a script that divides points in training and test groups, similar to v.kcv, but with points clustered in space. E.g.,

Create vector

grass.run_command(“v.random”, output=“testB”, npoints=10, overwrite=True)
grass.run_command(“v.db.addtable”, map=“testB”, columns=“X DOUBLE PRECISION,Y DOUBLE PRECISION,GR INTEGER”)
grass.run_command(“v.to.db”, map=“test”, option=“coor”, columns=“X,Y”)

Create groups

vectmap = ‘test’
cvals = array(grass.vector_db_select(vectmap, layer = int(1), columns = ‘X,Y’)[‘values’].values()).astype(np.float)
centroids,_ = kmeans(cvals,2)
idx,_ = vq(cvals,centroids)

write results to tabel

Now I would like to write idx to the column ‘GR’ in the attribute table of ‘test’.

p.s. I am first creating the XY columns now, but is there a function to get the coordinates (cvals) in pygrass directly?

I don’t fully understand the example,

Thanks for the quick response. I basically have a list with values (idx in the example above) which I like to add as a column to the attribute table of an existing vector (point layer). The length of idx is equal to the number of rows in the attribute table. The solution of Anna seems like an elegant solution (and easier and more flexible than other solutions I tried using e.g., sqlite3). However, as I wrote, I end up with a vector without attribute table. You wrote that I “have to save the changes in the database out from your cycle, with: vectormap.table.conn.commit()”. I am, however, not sure what you mean with ‘out from your cycle’ or how to implement that.

probably after the for cycle ends you would call this ‘vectormap.table.conn.commit()’, if it doesn’t works, try to put it in the cycle (I am not sure what is supposed to work).

12

On 01/10/15 00:05, Paulo van Breugel wrote:

On 30-09-15 17:52, Paulo van Breugel wrote:

On 30-09-15 17:47, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 11:20 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

    On 30-09-15 16:18, Anna Petrášová wrote:

    On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel
    <p.vanbreugel@gmail.com <mailto:p.vanbreugel@gmail.com>> wrote:

        On Wed, Sep 30, 2015 at 11:53 AM, Pietro
        <peter.zamb@gmail.com> wrote:

            On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
            <p.vanbreugel@gmail.com> wrote:
            >
            > On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová
            <kratochanna@gmail.com>
            > wrote:
            >>
            >> On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
            >> <p.vanbreugel@gmail.com> wrote:
            >>>
            >>> This must be a very basic question, but I can't find
            an easy/direct way
            >>> to do this. In python, if I have an array with
            values with a length equal to
            >>> the number of rows in an attribute table of a
            (point) vector layer, how can
            >>> I write those values to a new column in that
            attribute table. I can of
            >>> course first create the column, but than how to
            update that column with the
            >>> values in the array?
            >>
            >> it should be pretty easy to do with pygrass,
            unfortunately there is no
            >> example on assigning attributes in the official
            documentation [1], but it
            >> should be pretty easy, something like that (not tested):
            >>
            >> with VectorTopo('myvector', mode='w') as vectormap:
            >> for feature in vectormap:
            >> feature.attrs['mycolumn'] = value
            >>
            > Thanks, but that seems to write the vector back
            without attribute table

            You have to save the changes in the database out from
            your cycle, with:

            vectormap.table.conn.commit()

        Thanks Pietro. I am, however, not sure I understand (I tried
        to use it, but thanks to my limited experience in Python /
        pygrass not much luck). Just to be more specific, I am
        trying to create a script that divides points in training
        and test groups, similar to v.kcv, but with points clustered
        in space. E.g.,

        # Create vector
        grass.run_command("v.random", output="testB", npoints=10,
        overwrite=True)
        grass.run_command("v.db.addtable", map="testB", columns="X
        DOUBLE PRECISION,Y DOUBLE PRECISION,GR INTEGER")
        grass.run_command("v.to.db", map="test", option="coor",
        columns="X,Y")

        # Create groups
        vectmap = 'test'
        cvals = array(grass.vector_db_select(vectmap, layer =
        int(1), columns = 'X,Y')['values'].values()).astype(np.float)
        centroids,_ = kmeans(cvals,2)
        idx,_ = vq(cvals,centroids)

        # write results to tabel
        Now I would like to write idx to the column 'GR' in the
        attribute table of 'test'.

        p.s. I am first creating the XY columns now, but is there a
        function to get the coordinates (cvals) in pygrass directly?

    I don't fully understand the example,

    Thanks for the quick response. I basically have a list with
    values (idx in the example above) which I like to add as a column
    to the attribute table of an existing vector (point layer). The
    length of idx is equal to the number of rows in the attribute
    table. The solution of Anna seems like an elegant solution (and
    easier and more flexible than other solutions I tried using e.g.,
    sqlite3). However, as I wrote, I end up with a vector without
    attribute table. You wrote that I "have to save the changes in
    the database out from your cycle, with:
    vectormap.table.conn.commit()". I am, however, not sure what you
    mean with 'out from your cycle' or how to implement that.

probably after the for cycle ends you would call this
'vectormap.table.conn.commit()', if it doesn't works, try to put it
in the cycle (I am not sure what is supposed to work).

I did try both, with no luck so-far. I'll give it another try though,
perhaps I did something else wrong.

Tried out again, but after running the code below, I end up with an
empty vector layer (no points and no attribute table). Same if I put the
vectormap.table.conn.commit() in the for cycle.

import grass.script as grass

from grass.pygrass.vector import VectorTopo

grass.run_command("v.random", output="testB", npoints=10, overwrite=True)

grass.run_command("v.db.addtable", map="testB", columns="GR INTEGER")

with VectorTopo('testB', mode='w') as vectormap:

     for feature in vectormap:

         feature.attrs['GR'] = 9

         vectormap.table.conn.commit()

Just a wild guess, here without looking at the library code, but could it be that it should be

VectorTopo('testB', mode='a')

instead of

VectorTopo('testB', mode='w')

Moritz

13

Hi Paulo,
I had the same problem some time ago. I am using pandas for a lot of things and discovered the quickest way to append a column to a vector table is using pandas’ .to_sql dataframe method. Here is my full function for it:

import pandas as pa

dataframe = pa.DataFrame( ... )

def appendColumns(vecttbl,dataframe,join=None,layer=1):

    '''Upload a DataFrame (Series should be first converted into a sql compliant datafram)

    to a grass vector table. Join is a valid column in vecttbl or if None take categories.

    Always joins on df.index, which is not uploaded.'''

    # get sqlite.db info

    db = grass.vector_db(vecttbl)[layer]

    con = sqlite3.connect(db['database'])

    # decide join indeces

    itbl = {False:db['key'],True:join}[join!=None]

    # make series to dataframe

    dataframe.to_sql('pandas_temp',con,if_exists='replace',index_label='ix') # index label?

    # join columns to vecttbl

    grass.run_command('v.db.join',map=vecttbl,column=itbl,

                       other_table='pandas_temp',other_column='ix',

                       subset_columns=','.join(map(str,dataframe.columns)))

    # remove temp table

    cur = con.cursor()

    cur.execute('DROP TABLE IF EXISTS pandas_temp')

    con.commit(); con.close()

    return

The dataframe.to_sql line is essentially all you need, the rest of the function is just prepping.
Hope it helps,
Michel

···

On 10/01/2015 12:05 AM, Paulo van Breugel wrote:

Tried out again, but after running the code below, I end up with an empty vector layer (no points and no attribute table). Same if I put the vectormap.table.conn.commit() in the for cycle.

On 30-09-15 17:52, Paulo van Breugel wrote:

I did try both, with no luck so-far. I’ll give it another try though, perhaps I did something else wrong.

On 30-09-15 17:47, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 11:20 AM, Paulo van Breugel <p.vanbreugel@gmail.com> wrote:

On 30-09-15 16:18, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel <p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 11:53 AM, Pietro <peter.zamb@gmail.com> wrote:

On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com>
wrote:

On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

This must be a very basic question, but I can’t find an easy/direct way
to do this. In python, if I have an array with values with a length equal to
the number of rows in an attribute table of a (point) vector layer, how can
I write those values to a new column in that attribute table. I can of
course first create the column, but than how to update that column with the
values in the array?

it should be pretty easy to do with pygrass, unfortunately there is no
example on assigning attributes in the official documentation [1], but it
should be pretty easy, something like that (not tested):

with VectorTopo(‘myvector’, mode=‘w’) as vectormap:
for feature in vectormap:
feature.attrs[‘mycolumn’] = value

Thanks, but that seems to write the vector back without attribute table

You have to save the changes in the database out from your cycle, with:

vectormap.table.conn.commit()

Thanks Pietro. I am, however, not sure I understand (I tried to use it, but thanks to my limited experience in Python / pygrass not much luck). Just to be more specific, I am trying to create a script that divides points in training and test groups, similar to v.kcv, but with points clustered in space. E.g.,

Create vector

grass.run_command(“v.random”, output=“testB”, npoints=10, overwrite=True)
grass.run_command(“v.db.addtable”, map=“testB”, columns=“X DOUBLE PRECISION,Y DOUBLE PRECISION,GR INTEGER”)
grass.run_command(“v.to.db”, map=“test”, option=“coor”, columns=“X,Y”)

Create groups

vectmap = ‘test’
cvals = array(grass.vector_db_select(vectmap, layer = int(1), columns = ‘X,Y’)[‘values’].values()).astype(np.float)
centroids,_ = kmeans(cvals,2)
idx,_ = vq(cvals,centroids)

write results to tabel

Now I would like to write idx to the column ‘GR’ in the attribute table of ‘test’.

p.s. I am first creating the XY columns now, but is there a function to get the coordinates (cvals) in pygrass directly?

I don’t fully understand the example,

Thanks for the quick response. I basically have a list with values (idx in the example above) which I like to add as a column to the attribute table of an existing vector (point layer). The length of idx is equal to the number of rows in the attribute table. The solution of Anna seems like an elegant solution (and easier and more flexible than other solutions I tried using e.g., sqlite3). However, as I wrote, I end up with a vector without attribute table. You wrote that I “have to save the changes in the database out from your cycle, with: vectormap.table.conn.commit()”. I am, however, not sure what you mean with ‘out from your cycle’ or how to implement that.

probably after the for cycle ends you would call this ‘vectormap.table.conn.commit()’, if it doesn’t works, try to put it in the cycle (I am not sure what is supposed to work).

import grass.script as grass

from grass.pygrass.vector import VectorTopo

grass.run_command("v.random", output="testB", npoints=10, overwrite=True)

grass.run_command("v.db.addtable", map="testB", columns="GR INTEGER")

with VectorTopo('testB', mode='w') as vectormap:

    for feature in vectormap:

        feature.attrs['GR'] = 9

        vectormap.table.conn.commit()

but yes, you can get coordinates:

with VectorTopo(‘myvector’, mode=‘w’) as vectormap:
for feature in vectormap:
print feature.x

print feature.y

Great, thanks. I did not find this in the manual. If it is not there, perhaps it would be something worth including? I would not mind providing a text, but I am not sure what would be the best way to do that.

Technically, it’s in the manual:
https://grass.osgeo.org/grass70/manuals/libpython/pygrass_vector.html#geometry-classes

but we are lacking more examples of often used constructions. Some other examples are available in the recent workshop we did:

https://github.com/wenzeslaus/python-grass-addon/blob/master/02_pygrass_library.ipynb

Thanks, I will have a look at it.

Pietro

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14

On 01-10-15 09:15, Moritz Lennert wrote:

On 01/10/15 00:05, Paulo van Breugel wrote:

On 30-09-15 17:52, Paulo van Breugel wrote:

On 30-09-15 17:47, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 11:20 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

    On 30-09-15 16:18, Anna Petrášová wrote:

    On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel
    <p.vanbreugel@gmail.com <mailto:p.vanbreugel@gmail.com>> wrote:

        On Wed, Sep 30, 2015 at 11:53 AM, Pietro
        <peter.zamb@gmail.com> wrote:

            On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
            <p.vanbreugel@gmail.com> wrote:
            >
            > On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová
            <kratochanna@gmail.com>
            > wrote:
            >>
            >> On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
            >> <p.vanbreugel@gmail.com> wrote:
            >>>
            >>> This must be a very basic question, but I can't find
            an easy/direct way
            >>> to do this. In python, if I have an array with
            values with a length equal to
            >>> the number of rows in an attribute table of a
            (point) vector layer, how can
            >>> I write those values to a new column in that
            attribute table. I can of
            >>> course first create the column, but than how to
            update that column with the
            >>> values in the array?
            >>
            >> it should be pretty easy to do with pygrass,
            unfortunately there is no
            >> example on assigning attributes in the official
            documentation [1], but it
            >> should be pretty easy, something like that (not tested):
            >>
            >> with VectorTopo('myvector', mode='w') as vectormap:
            >> for feature in vectormap:
            >> feature.attrs['mycolumn'] = value
            >>
            > Thanks, but that seems to write the vector back
            without attribute table

            You have to save the changes in the database out from
            your cycle, with:

            vectormap.table.conn.commit()

        Thanks Pietro. I am, however, not sure I understand (I tried
        to use it, but thanks to my limited experience in Python /
        pygrass not much luck). Just to be more specific, I am
        trying to create a script that divides points in training
        and test groups, similar to v.kcv, but with points clustered
        in space. E.g.,

        # Create vector
        grass.run_command("v.random", output="testB", npoints=10,
        overwrite=True)
        grass.run_command("v.db.addtable", map="testB", columns="X
        DOUBLE PRECISION,Y DOUBLE PRECISION,GR INTEGER")
        grass.run_command("v.to.db", map="test", option="coor",
        columns="X,Y")

        # Create groups
        vectmap = 'test'
        cvals = array(grass.vector_db_select(vectmap, layer =
        int(1), columns = 'X,Y')['values'].values()).astype(np.float)
        centroids,_ = kmeans(cvals,2)
        idx,_ = vq(cvals,centroids)

        # write results to tabel
        Now I would like to write idx to the column 'GR' in the
        attribute table of 'test'.

        p.s. I am first creating the XY columns now, but is there a
        function to get the coordinates (cvals) in pygrass directly?

    I don't fully understand the example,

    Thanks for the quick response. I basically have a list with
    values (idx in the example above) which I like to add as a column
    to the attribute table of an existing vector (point layer). The
    length of idx is equal to the number of rows in the attribute
    table. The solution of Anna seems like an elegant solution (and
    easier and more flexible than other solutions I tried using e.g.,
    sqlite3). However, as I wrote, I end up with a vector without
    attribute table. You wrote that I "have to save the changes in
    the database out from your cycle, with:
    vectormap.table.conn.commit()". I am, however, not sure what you
    mean with 'out from your cycle' or how to implement that.

probably after the for cycle ends you would call this
'vectormap.table.conn.commit()', if it doesn't works, try to put it
in the cycle (I am not sure what is supposed to work).

I did try both, with no luck so-far. I'll give it another try though,
perhaps I did something else wrong.

Tried out again, but after running the code below, I end up with an
empty vector layer (no points and no attribute table). Same if I put the
vectormap.table.conn.commit() in the for cycle.

import grass.script as grass

from grass.pygrass.vector import VectorTopo

grass.run_command("v.random", output="testB", npoints=10, overwrite=True)

grass.run_command("v.db.addtable", map="testB", columns="GR INTEGER")

with VectorTopo('testB', mode='w') as vectormap:

     for feature in vectormap:

         feature.attrs['GR'] = 9

         vectormap.table.conn.commit()

Just a wild guess, here without looking at the library code, but could it be that it should be

VectorTopo('testB', mode='a')

instead of

VectorTopo('testB', mode='w')

That is not a valid option; "Mode not supported. Use one of: 'r', 'rw', 'w' ". This did make me try again though, but now with mode='rw', which worked:

idx= array([0, 1, 0, 0, 0, 0, 1, 1, 0, 1])

i = -1

with VectorTopo('testB', mode='rw') as vectormap:

     for feature in vectormap:

         i = i + 1

         feature.attrs['GR'] = idx[i]

     vectormap.table.conn.commit()

Thanks all for the help/advices

Moritz

15

Hi Michel,

Thanks! I think I got the suggestions by Anna, Pietro and Moritz working, but this certainly looks like an handy function to have at hand. One possible disadvantage if using this in a script to be shared with others is that it would add an dependency on pandas, wouldn’t it?

Cheers,

Paulo

···

On 01-10-15 10:01, Michel Wortmann wrote:

Hi Paulo,
I had the same problem some time ago. I am using pandas for a lot of things and discovered the quickest way to append a column to a vector table is using pandas’ .to_sql dataframe method. Here is my full function for it:

import pandas as pa

dataframe = pa.DataFrame( ... )

def appendColumns(vecttbl,dataframe,join=None,layer=1):

    '''Upload a DataFrame (Series should be first converted into a sql compliant datafram)

    to a grass vector table. Join is a valid column in vecttbl or if None take categories.

    Always joins on df.index, which is not uploaded.'''

    # get sqlite.db info

    db = grass.vector_db(vecttbl)[layer]

    con = sqlite3.connect(db['database'])

    # decide join indeces

    itbl = {False:db['key'],True:join}[join!=None]

    # make series to dataframe

    dataframe.to_sql('pandas_temp',con,if_exists='replace',index_label='ix') # index label?

    # join columns to vecttbl

    grass.run_command('v.db.join',map=vecttbl,column=itbl,

                       other_table='pandas_temp',other_column='ix',

                       subset_columns=','.join(map(str,dataframe.columns)))

    # remove temp table

    cur = con.cursor()

    cur.execute('DROP TABLE IF EXISTS pandas_temp')

    con.commit(); con.close()

    return

The dataframe.to_sql line is essentially all you need, the rest of the function is just prepping.
Hope it helps,
Michel

On 10/01/2015 12:05 AM, Paulo van Breugel wrote:

Tried out again, but after running the code below, I end up with an empty vector layer (no points and no attribute table). Same if I put the vectormap.table.conn.commit() in the for cycle.

On 30-09-15 17:52, Paulo van Breugel wrote:

I did try both, with no luck so-far. I’ll give it another try though, perhaps I did something else wrong.

On 30-09-15 17:47, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 11:20 AM, Paulo van Breugel <p.vanbreugel@gmail.com> wrote:

On 30-09-15 16:18, Anna Petrášová wrote:

On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel <p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 11:53 AM, Pietro <peter.zamb@gmail.com> wrote:

On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna@gmail.com>
wrote:

On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
<p.vanbreugel@gmail.com> wrote:

This must be a very basic question, but I can’t find an easy/direct way
to do this. In python, if I have an array with values with a length equal to
the number of rows in an attribute table of a (point) vector layer, how can
I write those values to a new column in that attribute table. I can of
course first create the column, but than how to update that column with the
values in the array?

it should be pretty easy to do with pygrass, unfortunately there is no
example on assigning attributes in the official documentation [1], but it
should be pretty easy, something like that (not tested):

with VectorTopo(‘myvector’, mode=‘w’) as vectormap:
for feature in vectormap:
feature.attrs[‘mycolumn’] = value

Thanks, but that seems to write the vector back without attribute table

You have to save the changes in the database out from your cycle, with:

vectormap.table.conn.commit()

Thanks Pietro. I am, however, not sure I understand (I tried to use it, but thanks to my limited experience in Python / pygrass not much luck). Just to be more specific, I am trying to create a script that divides points in training and test groups, similar to v.kcv, but with points clustered in space. E.g.,

Create vector

grass.run_command(“v.random”, output=“testB”, npoints=10, overwrite=True)
grass.run_command(“v.db.addtable”, map=“testB”, columns=“X DOUBLE PRECISION,Y DOUBLE PRECISION,GR INTEGER”)
grass.run_command(“v.to.db”, map=“test”, option=“coor”, columns=“X,Y”)

Create groups

vectmap = ‘test’
cvals = array(grass.vector_db_select(vectmap, layer = int(1), columns = ‘X,Y’)[‘values’].values()).astype(np.float)
centroids,_ = kmeans(cvals,2)
idx,_ = vq(cvals,centroids)

write results to tabel

Now I would like to write idx to the column ‘GR’ in the attribute table of ‘test’.

p.s. I am first creating the XY columns now, but is there a function to get the coordinates (cvals) in pygrass directly?

I don’t fully understand the example,

Thanks for the quick response. I basically have a list with values (idx in the example above) which I like to add as a column to the attribute table of an existing vector (point layer). The length of idx is equal to the number of rows in the attribute table. The solution of Anna seems like an elegant solution (and easier and more flexible than other solutions I tried using e.g., sqlite3). However, as I wrote, I end up with a vector without attribute table. You wrote that I “have to save the changes in the database out from your cycle, with: vectormap.table.conn.commit()”. I am, however, not sure what you mean with ‘out from your cycle’ or how to implement that.

probably after the for cycle ends you would call this ‘vectormap.table.conn.commit()’, if it doesn’t works, try to put it in the cycle (I am not sure what is supposed to work).

import grass.script as grass

from grass.pygrass.vector import VectorTopo

grass.run_command("v.random", output="testB", npoints=10, overwrite=True)

grass.run_command("v.db.addtable", map="testB", columns="GR INTEGER")

with VectorTopo('testB', mode='w') as vectormap:

    for feature in vectormap:

        feature.attrs['GR'] = 9

        vectormap.table.conn.commit()

but yes, you can get coordinates:

with VectorTopo(‘myvector’, mode=‘w’) as vectormap:
for feature in vectormap:
print feature.x

print feature.y

Great, thanks. I did not find this in the manual. If it is not there, perhaps it would be something worth including? I would not mind providing a text, but I am not sure what would be the best way to do that.

Technically, it’s in the manual:
https://grass.osgeo.org/grass70/manuals/libpython/pygrass_vector.html#geometry-classes

but we are lacking more examples of often used constructions. Some other examples are available in the recent workshop we did:

https://github.com/wenzeslaus/python-grass-addon/blob/master/02_pygrass_library.ipynb

Thanks, I will have a look at it.

Pietro

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16

On Thu, Oct 1, 2015 at 11:28 AM, Paulo van Breugel <p.vanbreugel@gmail.com>
wrote:

with VectorTopo('testB', mode='rw') as vectormap:

At the and it makes sense. 'w' removes the vectors and starts over. 'rw'
keep the file but ones it for writing. Perhaps 'a' would make better sense
in this case. What the documentation says?

17

On Thu, Oct 1, 2015 at 10:01 AM, Michel Wortmann
<wortmann@pik-potsdam.de> wrote:

Hi Paulo,
I had the same problem some time ago. I am using pandas for a lot of things
and discovered the quickest way to append a column to a vector table is
using pandas' .to_sql dataframe method. Here is my full function for it:

To add a ne column on an existing vector map in pygrass: write:

{{{
if 'column_name' in vect.table.columns:
    vect.table.columns.drop('column_name')
vect.table.columns.add('column_name', 'column_type')
}}}

18

On it is mentioned that: “”". I think this description is somewhat ambiguous. Especially the mode=‘w’ is unclear to me; I could clearly not write new features to an (existing) vector layer. It might be nice to have a short one-line explanation of each of these options, explaining the difference.

···

On 01-10-15 17:49, Vaclav Petras wrote:

On Thu, Oct 1, 2015 at 11:28 AM, Paulo van Breugel <p.vanbreugel@gmail.com> wrote:

with VectorTopo(‘testB’, mode=‘rw’) as vectormap:

At the and it makes sense. ‘w’ removes the vectors and starts over. ‘rw’ keep the file but ones it for writing. Perhaps ‘a’ would make better sense in this case. What the documentation says?

https://grass.osgeo.org/grass70/manuals/libpython/pygrass_vector.html

The open() method supports a number of option arguments (see the Info documentation for a complete list). In particular, the mode argument can take a a value of r for reading, w for writing, or rw for reading/writing." […] If a vector map is opened with the mode w or rw, then the user can write new features to the dataset