[GRASS-dev] Python equivalent of piping results from one function to another

[Paulo van Breugel]

On 19-10-15 07:12, Pietro wrote:
> Dear Paulo,
>
>
> On Sun, Oct 18, 2015 at 8:03 PM, Paulo van Breugel
> <p.vanbreugel at gmail.com> wrote:
>> What would be the python equivalent of:
>>
>> r.category mymap | r.category mymap rules=-
> Let's split the problem in two step.
>
> First capture the output of a module (the python equivalent of bash pipe):
>
> {{{
>>>> from subprocess import PIPE  # from the standard library import PIPE costant
>>>> from grass.pygrass.modules import Module  # import the Module class from pygrass
>>>> rcat = Module('r.category', 'landuse', stdout_=PIPE)  # use the spacial parameter stdout_
>>>> rcat.outputs.stdout   # the stdout is an attribute of the instanced class
> '1\tdeveloped\n2\tagriculture\n3\therbaceous\n4\tshrubland\n5\tforest\n6\twater\n7\tsediment\n'
>>>> print(rcat.outputs.stdout)
> 1       developed
> 2       agriculture
> 3       herbaceous
> 4       shrubland
> 5       forest
> 6       water
> 7       sediment
> }}}
>
> Second provide a string as stdinput of a module:
>
> {{{
>>>> Module('r.category', 'mymap', rules='-', stdin_='1\tdeveloped\n2\tagriculture\n3\therbaceous\n4\tshrubland\n5\tforest\n6\twater\n7\tsediment\n')
> Module('r.category')
> }}}
>
> or to do both things in one line:
>
> {{{
> Module('r.category', 'mymap', rules='-', stdin_=Module('r.category',
> 'landuse', stdout_=PIPE).outputs.stdout)
> }}}
>
> You can use a shourtcut to have a cleaner syntax close to bash:
>
> {{{
>>>> from grass.pygrass.modules.shortcuts import raster as r
>>>> r.category('mymap', rules='-', stdin_=r.category('landuse', stdout_=PIPE).outputs.stdout)
> Module('r.category')
> }}}

Dear Pietro, this is brilliant, thanks a  lot!
>
> There is an example in the documentation using r.colors but it is not
> rendered corectly [0]
I tried out the examples on that page, but I couldn't get it all to 
work. For example, there are some examples that use of "stdout_=PIPE" 
and stdin_=PIPE. I though that might be what I was looking for, but 
after trying (and failing) to run the examples I basically do not 
understand what is done there. The solution you describe above is 
actually more easy to intuitively understand. It would be a great 
example to include on the manual page.

About the manual pages, although very detailed, they are sometimes a bit 
hard to understand (perhaps mostly for non-programmers like me), and 
could benefit from worked out examples like you provide above. I would 
not mind providing input, but lack a proper understanding of the code. 
On the other hand, I have had feedback with great examples / 
explanations for two or three times now which I though would be great to 
include in the manual. What is the best place for such feedback / 
suggestions?

>
> Have fun!
>
> Pietro
>
> [0] https://grass.osgeo.org/grass71/manuals/libpython/pygrass.modules.interface.html#pygrass.modules.interface.module.Module



I had seen the example, but to be honest even after rereading it isn't 
all clear to me