Core Python Programming (2nd Edition)

Lines 13
The Unix startup line is followed by the module documentation string. Keep your documentation string
simple yet descriptive enough to be useful. Ours is a bit short, but so is this script. (We invite the reader
to take a look at the documentation string at the commencement of the cgi module in the standard
library for a seriously lengthy example of module documentation.)
Lines 56
We import the operating system (os) module next, and in line 6, we create a new local alias for the
linesep attribute of that module. By doing this, we can shorten the name of the variable and also speed
up access to it.
Lines 814
Core Tip: Use local variables to substitute for module attributes
Names like os.linesep require the interpreter to do two lookups: (1)
lookup os to find that it is a module, and (2) look up the linesep
attribute of that module. Because modules are also global variables,
we pay another penalty. If you use an attribute like this often in a
function, we recommend you alias it to a single local variable. Lookups
are much fasterlocal variables are always searched first before globals,
and we don't have attribute lookups either. This is one of the tricks in
making your programs faster: replace often-used (and name-lengthy)
module attributes with local references. Your code runs faster and has
less clutter with a shorter name.
In our code snippet, we do not have a function to show you an
example of using a local alias. Instead, we have a global alias, which is
halfway there. At least we do not have to perform two lookups to get
to the object.
If it is not apparent already, this is an "infinite loop," meaning we are presented with a body of code that
will repeat and run forever unless we exit the looplook for a break statement somewhere! The while true
conditional causes this to happen because while statements execute whenever its conditional expression
evaluates to Boolean true, and true is Boolean true.
Lines 1014 prompt the user for an unused filename, meaning that the filename entered should not be
the name of an already existing file. The raw_input() built-in function takes an argument to use as the
prompt to the user. The resulting string entered by the user is the return value of raw_input(), which in
this case gets assigned to fname.
If the user is unlucky enough to pick a name already in use, we notify the user and return the user to
the prompt to enter another (file)name. Note that os.path.exists() is a helper function in the os.path
(sub)module, which helps us make this determination. Only when a file with such a name does not exist,