There isn’t any big deal in your above snippet, but imagine a function with a few more arguments and quite a few more lines of code. Then you decide to rename your
data argument as
yadda, but miss one of the places it is used in the function’s body… Now
data refers to the global, and you start having weird behaviour – where you would have a much more obvious
NameError if you didn’t have a global name
Also remember that in Python everything is an object (including modules, classes and functions), so there’s no distinct namespaces for functions, modules or classes. Another scenario is that you import function
foo at the top of your module, and use it somewhere in your function body. Then you add a new argument to your function and named it – bad luck –
Finally, built-in functions and types also live in the same namespace and can be shadowed the same way.
None of this is much of a problem if you have short functions, good naming and a decent unit test coverage, but well, sometimes you have to maintain less than perfect code and being warned about such possible issues might help.