ESRI/arcpy frustrations [Python Case Sensitivity]

When I don’t use ESRI Python module for geoprocessing  – arcpy – every day, some of the idiosyncratic syntax of this (often not very Pythonic) module slips my mind. I was recently reminded of this when I ran into some problems with very short scripts. Turns out that since Python is case sensitive, you’re best bet as to which words to capitalize or camelcase, isn’t always correct. More frustrating though is the fact

>>> import arcpy
>>> sr = arcpy.Spatialreference()

Traceback (most recent call last):
File “<pyshell#4>”, line 1, in <module>
sr = arcpy.Spatialreference()
AttributeError: ‘module’ object has no attribute ‘Spatialreference’

>>> sr = arcpy.SpatialReference()
>>>

So ‘Spatialreference throws an error but ‘SpatialReference’ doesn’t. Remember to capitalize that ‘R’. ‘SpatialReference’ is a class of the arcpy module, and classes, according to PEP8, should be named using the CapWord (or CamelCase) spelling convention.
So what about this …


 >>> fc = "c:/temp/myfile.shp"
 >>> desc = arcpy.Describe(fc)
 >>> print desc.spatialreference.name
 NAD_1927_StatePlane_Texas_South_Central_FIPS_4204
 >>>

Apparently, here I don’t have to capitalize anything. ‘spatialreference’ works as the name of the property of dataset fc. But guess what ? So do:


>>> print desc.spatialreference.name
NAD_1927_StatePlane_Texas_South_Central_FIPS_4204
>>> print desc.SpatialReference.name
NAD_1927_StatePlane_Texas_South_Central_FIPS_4204
>>> print desc.SpatialReference.name
NAD_1927_StatePlane_Texas_South_Central_FIPS_4204

Well, that seems very inconsistent. Another one (taken from that tripped me up was:

fc = "c:/temp/dirSurvey.shp"
desc = arcpy.Describe(fc)
sr = desc.spatialReference
arcpy.CreateFeatureClass_management("C:/temp/","newFC","POLYLINE", "", "", "", sr)

Traceback (most recent call last):
  File “<pyshell#14>”, line 1, in <module>
    arcpy.CreateFeatureClass_management(“C:/temp/”,”newFC”,”POLYLINE”, “”, “”, “”, sr)
AttributeError: ‘module’ object has no attribute ‘CreateFeatureClass_management’
Apparently, this either used to be the correct name or some of the ESRI online documentation is wrong. But the method really is:

arcpy.CreateFeatureclass_management("C:/temp/","newFC","POLYLINE", "", "", "", sr)

Another one I keep getting wrong because I forget to capitalize is:

from arcpy import env
env.overwriteOutput = True # instead of overwriteoutput which would be wrong

To make a long story short, I’ve learned that when I can’t remember what the propert name is, I type:

>>> dir (arcpy) # or dir(arcpy.env)

and that gives me all the correct names for attributes and methods of arcpy that I need.

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4 Comments

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4 responses to “ESRI/arcpy frustrations [Python Case Sensitivity]

  1. Hi, I follow your blog.

    A question regarding this post, simply, and in part the problem is solved using a python interactive shell as ipython or bpython?. I do not know if I can use these tools in python with ArcGIS products.

    Greetings and congratulations for your blog.

  2. That’s why I’m using Eclipse with PyDev for my ArcPy development. I got tired of constantly having to look up the exact syntax for every method or property. With Eclipse I can reference the ArcPy library and I get full code completion with ArcPy. ArcPy.Create is all I type and then I get a list of all the functions to choose from. Selecting the function also puts place holders in the code for what the proper parameters to use are. It’s taken me 3 years but I finally have code completion with ArcGIS geoprocessing!

  3. David Viljoen

    You may also want to consider PyScripter (http://code.google.com/p/pyscripter). It an open-source Python IDE that has many of the features of Eclipse (code completion, debug windows, code templates, etc.) but has a much smaller footprint. One of the features I find useful is running the code with the Python Engine set to Internal so the arcpy module is imported once per debugging session not every time you run the script you are developing.

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