Core Python Programming (2nd Edition)

Earlier in this chapter, we described how the exec statement can be used with a file object argument to
read the contents of a Python script and execute it. This can be accomplished with the following code
segment:
f = open(filename, 'r')
exec f
f.close()
The three lines can be replaced by a single call to execfile():
execfile(filename)
Although the code above does execute a module, it does so only in its current execution environment (i.
e., its global and local namespace). There may be a desire to execute a module with a different set of
global and local namespaces instead of the default ones. The full syntax of execfile() is very similar to
that of eval():
execfile(filename, globals=globals(), locals=locals())
Like eval(), both globals and locals are optional and default to the executing environments'
namespaces if not given. If only globals is given, then locals defaults to globals. If provided, locals
can be any mapping object [an object defining/overriding __getitem__()], although before 2.4, it was
required to be a dictionary. Warning: be very careful with your local namespace (in terms of modifying
it). It is much safer to pass in a dummy "locals" dictionary and check for any side effects. Altering the
local namespace is not guaranteed by execfile()! See the Python Library Reference Manual's entry for
execfile() for more details.
14.4.3. Executing Modules as Scripts
A new command-line option (or switch) was added in Python 2.4 that allows you to directly execute a
module as a script from your shell or DOS prompt. When you are writing your own modules as scripts, it
is easy to execute them. From your working directory, you would just call your script on the command
line:
$ myScript.py # or $ python myScript.py
This is not as easy if you are dealing with modules that are part of the standard library, installed in sitepackages,
or just modules in packages, especially if they also share the same name as an existing
Python module. For example, let us say you wanted to run the free Web server that comes with Python
so that you can create and test Web pages and CGI scripts you wrote.
You would have to type something like the following at the command line: