Writing Multiple polygons to shapefile

[wjba...@gmail.com]

Joel,
I can create a shapefile with one (1) polygon and associated data using this:

import shapefile
w = shapefile.Writer(shapefile.POLYGON)
w.poly(parts)
w.field('FIRST_FLD','C','40')
w.field('SECOND_FLD','C','40')
w.record('First','Polygon')
w.save('c:\\Python27\\polygon')


Now I want to append more polygons to the same shape file. Nothing I have tried seems to be working.

import shapefile
w = shapefile.Writer(shapefile.POLYGON)
w.poly(parts)
w.field('FIRST_FLD','C','40')
w.field('SECOND_FLD','C','40')
w.record('First','Polygon')
w.save('c:\\Python27\\polygon')
#
# add second polygon (not working)
w.poly(parts2)
w.record('Second','Polygon')
w.save('c:\\Python27\\polygon')



Questions.I have...
1.
Do the w.field statements need to be repeated?
2.
Does the w.save statement close the shapefile?
3.
What separates the records?


Thanks
Bill

[Joel Lawhead]

Bill,

As soon as you call the "save()" method the shapefile is finalized to disk.  If you were to remove the first save call in your example, your code would create two polygons. 

To edit a shapefile after you have saved it you need to create a Reader object, read in the saved shapefile, transfer it to a Writer object, call the poly and record methods for the second polygon, and save again. 

Please see this blog post where I demonstrate exactly what you are trying to do using two polygons:

http://geospatialpython.com/2013/10/editing-shapefile.html

You may have noticed there is also an Editor class in the library.  That class is supposed to perform the above operation in a single object to save you some lines of code but people keep finding bugs so it's a work in progress.  The method above is extremely reliable so it's worth the extra steps. 

Let me know if how that goes for you or if you have any other questions. 

Thanks,

Joel

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Joel Lawhead, PMP
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[wjba...@gmail.com]

Joel,

I got my Autocad To Shape (ats.py) program working. I start in Autocad using Autolisp/OpenDCL (ats.lsp). I use a dialog box to select multiple polylines and enter the field information. ATS.lsp (autolisp program) then exports a text file that can be read by Python.

The Python program then reads the text file, translates/rotates the State Plane points using Corpscon6.dll.
Then it imports your shapefile.py that creates the shapefile.

ats.py also creates a prj file.

ArcGIS Explorer will add the files with no problems (ArcGis Coordinate system has to match what is specified in the prj file).

Thanks for the help and the shapefile.py
Bill

[Joel Lawhead]

Wow - that's neat!  There is nothing I love more than process automation.  

How does Python access the dll? Is it ctypes or the win32 package?

- Joel

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[wjba...@gmail.com]

I am using ctypes for the calls to Corpscon6.dll. I was not aware of the win32 package.

This is my first python program ( that does anything).

Getting these programs to interface with each other is a chore. I struggled for days until I found that Python file.readline() adds a '\n' that was in my variable I was sending to shapefile.py.

[Joel Lawhead]

Ctypes is the "right" way most 95% of the time. I was just curious. Not bad for your first script!

On Wednesday, December 11, 2013, wrote:

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