What is the benefit to define a function in a function in python?

Question

I encountered this piece of python code (pasted below) on effbot and I was wondering:

Why defining a function within a function?

import re, htmlentitydefs

##
# Removes HTML or XML character references and entities from a text string.
#
# @param text The HTML (or XML) source text.
# @return The plain text, as a Unicode string, if necessary.

def unescape(text):
    def fixup(m):
        text = m.group(0)
        if text[:2] == "&#":
            # character reference
            try:
                if text[:3] == "&#x":
                    return unichr(int(text[3:-1], 16))
                else:
                    return unichr(int(text[2:-1]))
            except ValueError:
                pass
        else:
            # named entity
            try:
                text = unichr(htmlentitydefs.name2codepoint[text[1:-1]])
            except KeyError:
                pass
        return text # leave as is
    return re.sub("(?s)<[^>]*>|&#?\w+;", fixup, text)

score 72 · Accepted Answer · answered Jul 22 '11 at 14:07

72

Why defining a function within a function?

To keep it isolated. It's only used in this one place. Why define it more globally when it's used locally?

answered Jul 22 '11 at 14:07

S.Lott

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It is pretty common to define inner functions for using closures but It seems the current case is the one you mentioned (and it is a good reason as well). Ergo, +1 – brandizzi Jul 22 '11 at 14:21

score 8 · Answer 2 · answered Jul 22 '11 at 14:09

8

It's just another way of breaking down a large function into smaller pieces without polluting the global namespace with another function name. Quite often the inner function isn't a stand-alone so doesn't rightfully belong in the global namespace.

answered Jul 22 '11 at 14:09

John Gaines Jr.

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Roman Bodnarchuk · Answer 3 · 2011-07-22T14:19:19.400

7

Often the main reason of such code is function closures. It is powerful thing that is applicable not only to Python. E.g. JavaScript gains a lot from them.

Some points about closures in Python - closures-in-python.

edited Jul 22 '11 at 14:19

answered Jul 22 '11 at 14:09

Roman Bodnarchuk

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What is the benefit to define a function in a function in python?

3 Answers3

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