PyMOTW: tabnanny - Indentation validator

By Doug Hellmann
March 7, 2010

tabnanny – Indentation validator

Purpose:Scan Python source code looking for suspicious indentation.
Python Version:2.1.3 and later

Consistent use of indentation is important in a langauge like Python, where white-space is significant. The tabnanny module provides a scanner to report on “ambiguous” use of indentation.

Running from the Command Line

The simplest way to use tabnanny is to run it from the command line, passing the names of files to check. If you pass directory names, the directories are scanned recursively to find .py files to check.

When I ran tabnanny across the PyMOTW source code, I found one old module with tabs instead of spaces:

$ python -m tabnanny .
./PyMOTW/Queue/fetch_podcasts.py 78 "\t\tfor enclosure in entry.get('enclosures', []):\n"

Sure enough, line 78 of fetch_podcasts.py had two tabs instead of 8 spaces. I didn’t see this by looking at it in my editor because I have my tabstops set to 4 spaces, so visually there was no difference.

for enclosure in entry.get('enclosures', []):
    print 'Queuing:', enclosure['url']
    enclosure_queue.put(enclosure['url'])

Correcting line 78 and running tabnanny again showed another error on line 79. One last problem showed up on line 80.

If you want to scan files, but not see the details about the error, you can use the -q option to suppress all information except the filename.

$ python -m tabnanny -q .
./PyMOTW/Queue/fetch_podcasts.py

To see more information about the files being scanned, use the -v option.

$ python -m tabnanny -v ./PyMOTW/Queue
'./PyMOTW/Queue': listing directory
'./PyMOTW/Queue/__init__.py': Clean bill of health.
'./PyMOTW/Queue/feedparser.py': Clean bill of health.
'./PyMOTW/Queue/fetch_podcasts.py': *** Line 78: trouble in tab city! ***
offending line: "\t\tfor enclosure in entry.get('enclosures', []):\n"
indent not greater e.g. at tab sizes 1, 2

Using within Your Program

As soon as I discovered the mistake in my Queue example, I decided I needed to add a tabnanny check to my PyMOTW build process. I created a tabcheck task in my pavement.py build script so I could run paver tabcheck and scan the code I’m working on for PyMOTW. This is possible because tabnanny exposes its check() function as a public API.

Here’s an example of using tabnanny that doesn’t require understanding Paver’s task definition decorators.

import sys
import tabnanny

# Turn on verbose mode
tabnanny.verbose = 1

for dirname in sys.argv[1:]:
    tabnanny.check(dirname)

And in action:

$ python tabnanny_check.py ../Queue
'../Queue': listing directory
'../Queue/__init__.py': Clean bill of health.
'../Queue/feedparser.py': Clean bill of health.
'../Queue/fetch_podcasts.py': *** Line 78: trouble in tab city! ***
offending line: "\t\tfor enclosure in entry.get('enclosures', []):\n"
indent not greater e.g. at tab sizes 1, 2

Note

If you run these examples against the PyMOTW code it won’t report the same errors, since I have fixed my code in this release.

See also

tabnanny
The standard library documentation for this module.
tokenize
Lexical scanner for Python source code.

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