Question

I am a newbie to Python, i am having some doubts on working of below mentioned code snippet from shutil module.

def ignore_patterns(*patterns):
    """Function that can be used as copytree() ignore parameter.

    Patterns is a sequence of glob-style patterns
    that are used to exclude files"""
    def _ignore_patterns(path, names):
        ignored_names = []
        for pattern in patterns:
            ignored_names.extend(fnmatch.filter(names, pattern))
        return set(ignored_names)
    return _ignore_patterns

When a call to shutil.copytree is made with ignore option set to ignore_patterns, then it calls ignore_patterns function and returns a function. My doubts are:

1) ignore_patterns when called will return the _ignore_pattern function reference. Now when this function will be called, how is it still accessing the “patterns” list? Once called function “ignore_patterns” has returned, list patterns which was created at its call should be available for its called scope only.

2) What is the signicance of underscore in the returned function _ignore_patterns function name?

Answer 1

This is called a closure, and it’s a general feature of languages that allow nested functions. Inner functions can close over variables in an outer scope, and will retain a reference to that name when they are called from outside the outer function.

The underscore is just to signify that _ignore_patterns is an inner function, while keeping the name of the returned function similar. It could be called anything you like.