Python del statement

问题:

Calling del on a variable in Python. Does this free the allocated memory immediately or still waiting for garbage collector to collect? Like in java, explicitly calling del has no effect on when the memory will be freed.

回答1:

The del statement doesn't reclaim memory. It removes a reference, which decrements the reference count on the value. If the count is zero, the memory can be reclaimed. CPython will reclaim the memory immediately, there's no need to wait for the garbage collector to run.

In fact, the garbage collector is only needed for reclaiming cyclic structures.

As Waleed Khan says in his comment, Python memory management just works, you don't have to worry about it.

回答2:

"Deletion of a name removes the binding of that name from the local or global namespace". No more, no less. It does nothing to the object the name pointed to, except decrementing its refcount, and if refcount is not zero, the object will not be collected even when GC runs.

回答3:

Also, the del statement seems to be a little bit faster than assigning None (similar to Java's style assigning null to a variable to free its memory ...).

To compare:

import time, math

def measure_del():
        start = time.time()
        for i in range(0,int(math.pow(10,8))):
                    a = "123"
                    del a # <--- !!!
        end = time.time()
        print(end-start)

def measure_none():
        start = time.time()
        for i in range(0,int(math.pow(10,8))):
                    a = "123"
                    a = None # <--- !!!
        end = time.time()
        print(end-start)

results in (running in idle3.4):

>>> measure_del()
3.9930295944213867
>>> measure_del()
3.7402305603027344
>>> measure_del()
3.8423104286193848
>>> measure_del()
3.753770351409912
>>> measure_del()
3.7772741317749023
>>> measure_del()
3.815058946609497

>>> measure_none()
4.052351236343384
>>> measure_none()
4.130320072174072
>>> measure_none()
4.082390069961548
>>> measure_none()
4.100180625915527
>>> measure_none()
4.071730375289917
>>> measure_none()
4.136169672012329