Can `del` make Python faster?

Question

As a programmer, I generally try to avoid the del statement because it is often an extra complication that Python program don't often need. However, when browsing the standard library (threading, os, etc...) and the pseudo-standard library (numpy, scipy, etc...) I see it used a non-zero amount of times, and I'd like to better understand when it is/isn't appropriate the del statement.

Specifically, I'm curious about the relationship between the Python del statement and the efficiency of a Python program. It seems to me that del might help a program run faster by reducing the amount of clutter lookup instructions need to sift through. However, I can also see a world where the extra instruction takes up more time than it saves.

My question is: does anyone have any interesting code snippets that demonstrate cases where del significantly changes the speed of the program? I'm most interested in cases where del improves the execution speed of a program, although non-trivial cases where del can really hurt are also interesting.

Answer 1

The main reason that standard Python libraries use del is not for speed but for namespace decluttering ("avoiding namespace pollution" is another term I believe I have seen for this). As user2357112 noted in a comment, it can also be used to break a traceback cycle.

Let's take a concrete example: line 58 of types.py in the cpython implementation reads:

del sys, _f, _g, _C, _c, # Not for export

If we look above, we find:

def _f(): pass
FunctionType = type(_f)
LambdaType = type(lambda: None)         # Same as FunctionType
CodeType = type(_f.__code__)
MappingProxyType = type(type.__dict__)
SimpleNamespace = type(sys.implementation)

def _g():
    yield 1
GeneratorType = type(_g())

_f and _g are two of the names being deled; as the comment says, they are "not for export".¹

You might think this is covered via:

__all__ = [n for n in globals() if n[:1] != '_']

(which is near the end of that same file), but as What's the python __all__ module level variable for? (and the linked Can someone explain __all__ in Python?) note, these affect the names exported via from types import *, rather than what's visible via import types; dir(types).

It's not necessary to clean up your module namespace, but doing so prevents people from sneaking into it and using undefined items. So it's good for a couple of purposes.

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¹Looks like someone forgot to update this to include _ag. _GeneratorWrapper is harder to hide, unfortunately.