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>>> money(123.456)

'$123.46'

>>> money(-9999999.99, 15)

'$ −9,999,999.99'

>>> X = 99999999999999999999

>>> '%s (%s)' % (commas(X), X)

'99,999,999,999,999,999,999 (99999999999999999999)'

Because this file uses the docstring feature introduced in Chapter 15, we can use the help function to explore its tools as well—it serves as a general-purpose tool:

>>> import formats

>>> help(formats)

Help on module formats:

NAME

formats

FILE

c:\misc\formats.py

DESCRIPTION

Various specialized string display formatting utilities.

Test me with canned self-test or command-line arguments.

FUNCTIONS

commas(N)

format positive integer-like N for display with

commas between digit groupings: xxx,yyy,zzz

money(N, width=0)

format number N for display with commas, 2 decimal digits,

leading $ and sign, and optional padding: $ -xxx,yyy.zz

You can use command-line arguments in similar ways to provide general inputs to scripts that may also package their code as functions and classes for reuse by importers. For more advanced command-line processing, be sure to see the getopt and optparse modules in Python’s standard library and manuals. In some scenarios, you might also use the built-in input function introduced in Chapter 3 and used in Chapter 10 to prompt the shell user for test inputs instead of pulling them from the command line.

* * *

Note

Also see Chapter 7’s discussion of the new {,d} string format method syntax that will be available in Python 3.1 and later; this formatting extension separates thousands groups with commas much like the code here. The module listed here, though, adds money formatting and serves as a manual alternative for comma insertion for Python versions before 3.1.

* * *

Changing the Module Search Path

In Chapter 21, we learned that the module search path is a list of directories that can be customized via the environment variable PYTHONPATH, and possibly via .pth files. What I haven’t shown you until now is how a Python program itself can actually change the search path by changing a built-in list called sys.path (the path attribute in the built-in sys module). sys.path is initialized on startup, but thereafter you can delete, append, and reset its components however you like:

>>> import sys

>>> sys.path

['', 'C:\\users', 'C:\\Windows\\system32\\python30.zip', ...more deleted...]

>>> sys.path.append('C:\\sourcedir') # Extend module search path

>>> import string # All imports search the new dir last

Once you’ve made such a change, it will impact future imports anywhere in the Python program, as all imports and all files share the single sys.path list. In fact, this list may be changed arbitrarily:

>>> sys.path = [r'd:\temp'] # Change module search path

>>> sys.path.append('c:\\lp4e\\examples') # For this process only

>>> sys.path

['d:\\temp', 'c:\\lp4e\\examples']

>>> import string

Traceback (most recent call last):

File "", line 1, in

ImportError: No module named string

Thus, you can use this technique to dynamically configure a search path inside a Python program. Be careful, though: if you delete a critical directory from the path, you may lose access to critical utilities. In the prior example, for instance, we no longer have access to the string module because we deleted the Python source library’s directory from the path.

Also, remember that such sys.path settings endure for only as long as the Python session or program (technically, process) that made them runs; they are not retained after Python exits. PYTHONPATH and .pth file path configurations live in the operating system instead of a running Python program, and so are more global: they are picked up by every program on your machine and live on after a program completes.

The as Extension for import and from

Both the import and from statements have been extended to allow an imported name to be given a different name in your script. The following import statement:

import modulename as name

is equivalent to:

import modulename

name