Question

I understand that del d[key] will delete the key-value pair, whereas d[key]=None only de-references the value.

However, in terms of memory management, is there any difference? Does setting a value None trigger garbage collection immediately, assuming that there is no other variable referencing this value?

I ran a little experiment:

In [74]: import sys
In [75]: a = {'a': 'blah'}

In [76]: sys.getsizeof(a)
Out[76]: 280

In [77]: a['a'] = None

In [79]: sys.getsizeof(a)
Out[79]: 280

In [80]: del a['a']

In [81]: sys.getsizeof(a)
Out[81]: 280

Not sure if the approach is valid, but it seems no difference in terms of the size of the dictionary at all. I must miss something here.

Answer 1

sys.getsizeof measures the size of the dict itself; not the size of the values it contains.

None is an object. It requires some memory.

To find the size of a dict including the size of the values it contains, you could use pympler:

In [26]: import pympler.asizeof as asizeof

In [27]: asizeof.asizeof({'a': None})
Out[27]: 168

In [28]: asizeof.asizeof({})
Out[28]: 136

In [29]: import sys

In [30]: 
In [31]: sys.getsizeof({})
Out[31]: 136

In [34]: sys.getsizeof({'a':None})
Out[34]: 136