Doug Hellmann

PyMOTW: File Access

2009-07-12T08:30:00.001-04:00

File Access

Python’s standard library includes a large range of tools for working with files, filenames, and file contents.

Filenames

The first step in working with files is to get the name of the file so you can operate on it. Python represents filenames as simple strings, but provides tools for building them from standard, platform-independent, components in os.path. List the contents of a directory with listdir() from os, or use glob to build a list of filenames from a pattern. Finer grained filtering of filenames is possible with fnmatch.

Meta-data

Once you know the name of the file, you may want to check other characteristics such as permissions or the file size using os.stat() and the constants in stat.

Reading Files

If you’re writing a filter application that processes text input line-by-line, fileinput provides an easy framework to get started. The fileinput API calls for you to iterate over the input() generator, processing each line as it is yielded. The generator handles parsing command line arguments for file names, or falling back to reading directly from sys.stdin. The result is a flexible tool your users can run directly on a file or as part of a pipeline.

If your app needs random access to files, linecache makes it easy to read lines by their line number. The contents of the file are maintained in a cache, so be careful of memory consumption.

Temporary Files

For cases where you need to create scratch files to hold data temporarily, or before moving it to a permanent location, tempfile will be very useful. It provides classes to create temporary files and directories safely and securely. Names are guaranteed not to collide, and include random components so they are not easily guessable.

Files and Directories

Frequently you need to work on a file as a whole, without worrying about what is in it. The shutil module includes high-level file operations such as copying files and directories, setting permissions, etc.

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Book Review: IronPython in Action

2009-07-06T08:05:00.001-04:00

IronPython in Action by Michael Foord and Christian Muirhead covers the version of Python built to run on Microsoft's CLR and explains how to use it with the .NET framework.

Disclaimer: I received a review copy of this book from Manning through the PyATL Book Club.

There are two target audiences for this book: experienced Python developers wanting to learn .NET, and experienced .NET developers wanting to learn Python. Both groups will find plenty of interesting material and learn a lot. After some relatively basic introductory chapters, the authors dig right in building a complex GUI application, and then implementing a web interface for the same desktop application.

Along the way they introduce topics such as different programming models in Python, navigating the MSDN documentation (very important for understanding the scope of features available in .NET), packaging your app for distribution under Windows, data persistence, XML parsing, design patterns, automated testing, system administration, relational databases, and two separate GUI libraries.

All of the code is clear, concise, and useful -- there are no fluffy, throw-away code snippets that fall short in the real world. While many of the examples given are specific to IronPython or .NET, the techniques being illustrated are definitely not.

I recommend this book for any Windows developer interested in learning about Python, and for Python developers looking into deploying an application under Windows. If you don't fall into either of those groups, I can still recommend that you pick up a copy for some excellent advice on general programming topics and the solid example code.

New command line interface to PyMOTW

2009-07-05T09:30:00.001-04:00

The 1.95 release of PyMOTW includes a command line interface to access the documentation for a module.

The package can be installed via easy_install or pip:

$ pip install PyMOTW
Downloading/unpacking PyMOTW
  Downloading PyMOTW-1.95.tar.gz (2.2Mb): 2.2Mb downloaded
  Running setup.py egg_info for package PyMOTW
    warning: no files found matching 'ChangeLog'
    warning: no files found matching '*.py' under directory 'sphinx/templates'
    no previously-included directories found matching 'utils'
Installing collected packages: PyMOTW
  Running setup.py install for PyMOTW
    changing mode of build/scripts-2.6/motw from 644 to 755
    warning: no files found matching 'ChangeLog'
    warning: no files found matching '*.py' under directory 'sphinx/templates'
    no previously-included directories found matching 'utils'
    changing mode of /Users/dhellmann/.virtualenvs/testpymotw/bin/motw to 755
Successfully installed PyMOTW

and then to use the command line interface, run motw.

$ motw -h
Usage: motw [options] 

Options:
  -h, --help  show this help message and exit
  -t, --text  Print plain-text version of help to stdout
  -w, --web   Open HTML version of help from web
  --html      Open HTML version of help from installed file

For example, motw abc opens the local version of this week's article. You can also use the "-w" option to go to my web site instead of reading the local version, so you always have the latest version of an article.