What is the difference between len() and sys.getsizeof() methods in python?

When I ran the below code I got 3 and 36 as the answers respectively.

x ="abd"
print len(x)
print sys.getsizeof(x)

Can someone explain to me what’s the difference between them ?


They are not the same thing at all.

len() queries for the number of items contained in a container. For a string that’s the number of characters:

Return the length (the number of items) of an object. The argument may be a sequence (string, tuple or list) or a mapping (dictionary).

sys.getsizeof() on the other hand returns the memory size of the object:

Return the size of an object in bytes. The object can be any type of object. All built-in objects will return correct results, but this does not have to hold true for third-party extensions as it is implementation specific.

Python string objects are not simple sequences of characters, 1 byte per character.

Specifically, the sys.getsizeof() function includes the garbage collector overhead if any:

getsizeof() calls the object’s __sizeof__ method and adds an additional garbage collector overhead if the object is managed by the garbage collector.

String objects do not need to be tracked (they cannot create circular references), but string objects do need more memory than just the bytes per character. In Python 2, __sizeof__ method returns (in C code):

Py_ssize_t res;
res = PyStringObject_SIZE + PyString_GET_SIZE(v) * Py_TYPE(v)->tp_itemsize;
return PyInt_FromSsize_t(res);

where PyStringObject_SIZE is the C struct header size for the type, PyString_GET_SIZE basically is the same as len() and Py_TYPE(v)->tp_itemsize is the per-character size. In Python 2.7, for byte strings, the size per character is 1, but it’s PyStringObject_SIZE that is confusing you; on my Mac that size is 37 bytes:

>>> sys.getsizeof('')

For unicode strings the per-character size goes up to 2 or 4 (depending on compilation options). On Python 3.3 and newer, Unicode strings take up between 1 and 4 bytes per character, depending on the contents of the string.

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