Which is better in python, del or delattr?

Question

This may be silly, but it's been nagging the back of my brain for a while.

Python gives us two built-in ways to delete attributes from objects, the del command word and the delattr built-in function. I prefer delattr because it I think its a bit more explicit:

del foo.bar
delattr(foo, "bar")

But I'm wondering if there might be under-the-hood differences between them.

Solution 1

The first is more efficient than the second. del foo.bar compiles to two bytecode instructions:

  2           0 LOAD_FAST                0 (foo)
              3 DELETE_ATTR              0 (bar)

whereas delattr(foo, "bar") takes five:

  2           0 LOAD_GLOBAL              0 (delattr)
              3 LOAD_FAST                0 (foo)
              6 LOAD_CONST               1 ('bar')
              9 CALL_FUNCTION            2
             12 POP_TOP             

This translates into the first running slightly faster (but it's not a huge difference – .15 μs on my machine).

Like the others have said, you should really only use the second form when the attribute that you're deleting is determined dynamically.

[Edited to show the bytecode instructions generated inside a function, where the compiler can use LOAD_FAST and LOAD_GLOBAL]

Solution 2

• del is more explicit and efficient;
• delattr allows dynamic attribute deleting.

Consider the following examples:

for name in ATTRIBUTES:
    delattr(obj, name)

or:

def _cleanup(self, name):
    """Do cleanup for an attribute"""
    value = getattr(self, name)
    self._pre_cleanup(name, value)
    delattr(self, name)
    self._post_cleanup(name, value)

You can't do it with del.

Solution 3

Unquestionably the former. In my view this is like asking whether foo.bar is better than getattr(foo, "bar"), and I don't think anyone is asking that question :)

Solution 4

It's really a matter of preference, but the first is probably preferable. I'd only use the second one if you don't know the name of the attribute that you're deleting ahead of time.

Solution 5

Just like getattr and setattr, delattr should only be used when the attribute name is unknown.

In that sense, it's roughly equivalent to several python features that are used to access built-in functionality at a lower level than you normally have available, such as __import__ instead of import and operator.add instead of +

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Comments

[Kenan Banks]

What tool did you use to generate this?

[Miles]

The dis module. You can run it from the command line using python -m dis and typing in some code, or disassemble a function with dis.dis().

[Jason Baker]

I'm sure there's at least one person out there that would prefer getattr(foo, "bar") over foo.bar. Granted, I wouldn't agree with them. But that one person is still enough to make it not unquestionably the former.

[Lennart Regebro]

Premature optimization is the root of all evil. ;-) But yes, you are right, of course.

[Sheep]

..so does it follow that the love of money is premature optimization?

[Phob]

@Jason Then nothing is "unquestionably better" than anything else by your interpretation of the phrase. I think this is a perfectly reasonable use of "unquestionably".

[Marc Gibbons]

getattr is preferable when there's a possibility that the property does not exist and you wish to set a default value without having to write try/except blocks. ie. gettattr(foo, "bar", None)

[mike rodent]

@John Fouhy only if you subscribe to both assertions. But both have stood the test of time.

[João dos Reis]

Premature optimization is a subset of the love of money, as it means you're trying to spend less on processing power :)

[ArtOfWarfare]

@MarcGibbons: Wait, that's a thing? I've always used the pattern if hasattr(obj, 'var') and obj.var == ...... I could just reduce this to, if getattr(obj, 'var', None) == ... in most cases! Slightly shorter and easier to read, I think.

[Mouscellaneous]

This isn't premature optimization but rather an empirical approach in making a decision as to which approach is better

[cheniel]

@ArtOfWarfare hasattr actually calls getattr

[user2859458]

I think it's important to note there are no Python programs where 150 ns will be significant in any way, so the time "efficiency difference" between the two methods should not be the determining factor in which to use. Rather, readability and if you need to pass a string or not should probably be the deciding factor.

[smac89]

You can do the second one with del. del self.__dict__[name], assuming of course no funny business going on with the meta class