How do I determine the size of an object in Python

Question

In C, we can find the size of an int, char, etc. I want to know how to get size of objects like a string, integer, etc. in Python.

Related question: How many bytes per element are there in a Python list (tuple)?

I am using an XML file which contains size fields that specify the size of value. I must parse this XML and do my coding. When I want to change the value of a particular field, I will check the size field of that value. Here I want to compare whether the new value that I'm gong to enter is of the same size as in XML. I need to check the size of new value. In case of a string I can say its the length. But in case of int, float, etc. I am confused.

Answer 1

How do I determine the size of an object in Python?

The answer, "Just use sys.getsizeof" is not a complete answer.

That answer does work for builtin objects directly, but it does not account for what those objects may contain, specifically, what types, such as tuples, lists, dicts, and sets contain. They can contain instances each other, as well as numbers, strings and other objects.

A More Complete Answer

Using 64 bit Python 3.6 from the Anaconda distribution, with sys.getsizeof, I have determined the minimum size of the following objects, and note that sets and dicts preallocate space so empty ones don't grow again until after a set amount (which may vary by implementation of the language):

Python 3:

Empty
Bytes  type        scaling notes
28     int         +4 bytes about every 30 powers of 2
37     bytes       +1 byte per additional byte
49     str         +1-4 per additional character (depending on max width)
48     tuple       +8 per additional item
64     list        +8 for each additional
224    set         5th increases to 736; 21nd, 2272; 85th, 8416; 341, 32992
240    dict        6th increases to 368; 22nd, 1184; 43rd, 2280; 86th, 4704; 171st, 9320
136    func def    does not include default args and other attrs
1056   class def   no slots 
56     class inst  has a __dict__ attr, same scaling as dict above
888    class def   with slots
16     __slots__   seems to store in mutable tuple-like structure
                   first slot grows to 48, and so on.

How do you interpret this? Well say you have a set with 10 items in it. If each item is 100 bytes each, how big is the whole data structure? The set is 736 itself because it has sized up one time to 736 bytes. Then you add the size of the items, so that's 1736 bytes in total

Some caveats for function and class definitions:

Note each class definition has a proxy __dict__ (48 bytes) structure for class attrs. Each slot has a descriptor (like a property) in the class definition.

Slotted instances start out with 48 bytes on their first element, and increase by 8 each additional. Only empty slotted objects have 16 bytes, and an instance with no data makes very little sense.

Also, each function definition has code objects, maybe docstrings, and other possible attributes, even a __dict__.

Python 2.7 analysis, confirmed with guppy.hpy and sys.getsizeof:

Bytes  type        empty + scaling notes
24     int         NA
28     long        NA
37     str         + 1 byte per additional character
52     unicode     + 4 bytes per additional character
56     tuple       + 8 bytes per additional item
72     list        + 32 for first, 8 for each additional
232    set         sixth item increases to 744; 22nd, 2280; 86th, 8424
280    dict        sixth item increases to 1048; 22nd, 3352; 86th, 12568 *
120    func def    does not include default args and other attrs
64     class inst  has a __dict__ attr, same scaling as dict above
16     __slots__   class with slots has no dict, seems to store in 
                   mutable tuple-like structure.
904    class def   has a proxy __dict__ structure for class attrs
104    old class   makes sense, less stuff, has real dict though.

Note that dictionaries (but not sets) got a more compact representation in Python 3.6

I think 8 bytes per additional item to reference makes a lot of sense on a 64 bit machine. Those 8 bytes point to the place in memory the contained item is at. The 4 bytes are fixed width for unicode in Python 2, if I recall correctly, but in Python 3, str becomes a unicode of width equal to the max width of the characters.

(And for more on slots, see this answer )

Recursive Visitor for a More Complete Function

To cover most of these types, I wrote this recursive function to try to estimate the size of most Python objects, including most builtins, types in the collections module, and custom types (slotted and otherwise):

import sys
from numbers import Number
from collections import Set, Mapping, deque

try: # Python 2
    zero_depth_bases = (basestring, Number, xrange, bytearray)
    iteritems = 'iteritems'
except NameError: # Python 3
    zero_depth_bases = (str, bytes, Number, range, bytearray)
    iteritems = 'items'

def getsize(obj_0):
    """Recursively iterate to sum size of object & members."""
    _seen_ids = set()
    def inner(obj):
        obj_id = id(obj)
        if obj_id in _seen_ids:
            return 0
        _seen_ids.add(obj_id)
        size = sys.getsizeof(obj)
        if isinstance(obj, zero_depth_bases):
            pass # bypass remaining control flow and return
        elif isinstance(obj, (tuple, list, Set, deque)):
            size += sum(inner(i) for i in obj)
        elif isinstance(obj, Mapping) or hasattr(obj, iteritems):
            size += sum(inner(k) + inner(v) for k, v in getattr(obj, iteritems)())
        # Check for custom object instances - may subclass above too
        if hasattr(obj, '__dict__'):
            size += inner(vars(obj))
        if hasattr(obj, '__slots__'): # can have __slots__ with __dict__
            size += sum(inner(getattr(obj, s)) for s in obj.__slots__ if hasattr(obj, s))
        return size
    return inner(obj_0)

And I tested it rather casually (I should unittest it):

>>> getsize(['a', tuple('bcd'), Foo()])
344
>>> getsize(Foo())
16
>>> getsize(tuple('bcd'))
194
>>> getsize(['a', tuple('bcd'), Foo(), {'foo': 'bar', 'baz': 'bar'}])
752
>>> getsize({'foo': 'bar', 'baz': 'bar'})
400
>>> getsize({})
280
>>> getsize({'foo':'bar'})
360
>>> getsize('foo')
40
>>> class Bar():
...     def baz():
...         pass
>>> getsize(Bar())
352
>>> getsize(Bar().__dict__)
280
>>> sys.getsizeof(Bar())
72
>>> getsize(Bar.__dict__)
872
>>> sys.getsizeof(Bar.__dict__)
280

It kind of breaks down on class definitions and function definitions because I don't go after all of their attributes, but since they should only exist once in memory for the process, their size really doesn't matter too much.

Answer 2

For numpy arrays, getsizeof doesn't work - for me it always returns 40 for some reason:

from pylab import *
from sys import getsizeof
A = rand(10)
B = rand(10000)

Then (in ipython):

In [64]: getsizeof(A)
Out[64]: 40

In [65]: getsizeof(B)
Out[65]: 40

Happily, though:

In [66]: A.nbytes
Out[66]: 80

In [67]: B.nbytes
Out[67]: 80000

Answer 3

Having run into this problem many times myself, I wrote up a small function (inspired by @aaron-hall's answer) & tests that does what I would have expected sys.getsizeof to do:

https://github.com/bosswissam/pysize

If you're interested in the backstory, here it is

EDIT: Attaching the code below for easy reference. To see the most up-to-date code, please check the github link.

    import sys

    def get_size(obj, seen=None):
        """Recursively finds size of objects"""
        size = sys.getsizeof(obj)
        if seen is None:
            seen = set()
        obj_id = id(obj)
        if obj_id in seen:
            return 0
        # Important mark as seen *before* entering recursion to gracefully handle
        # self-referential objects
        seen.add(obj_id)
        if isinstance(obj, dict):
            size += sum([get_size(v, seen) for v in obj.values()])
            size += sum([get_size(k, seen) for k in obj.keys()])
        elif hasattr(obj, '__dict__'):
            size += get_size(obj.__dict__, seen)
        elif hasattr(obj, '__iter__') and not isinstance(obj, (str, bytes, bytearray)):
            size += sum([get_size(i, seen) for i in obj])
        return size

Answer 4

First: an answer.

import sys

try: print sys.getsizeof(object)
except AttributeError:
    print "sys.getsizeof exists in Python ≥2.6"

Discussion:
In Python, you cannot ever access "direct" memory addresses. Why, then, would you need or want to know how many such addresses are occupied by a given object?? It's a question that's entirely inappropriate at that level of abstraction. When you're painting your house, you don't ask what frequencies of light are absorbed or reflected by each of the constituent atoms within the paint, you just ask what color it is -- the details of the physical characteristics that create that color are beside the point. Similarly, the number of bytes of memory that a given Python object occupies is beside the point.

So, why are you trying to use Python to write C code? :)

Answer 5

This can be more complicated than it looks depending on how you want to count things. For instance, if you have a list of ints, do you want the size of the list containing the references to the ints? (ie. list only, not what is contained in it), or do you want to include the actual data pointed to, in which case you need to deal with duplicate references, and how to prevent double-counting when two objects contain references to the same object.

You may want to take a look at one of the python memory profilers, such as pysizer to see if they meet your needs.

Answer 6

Just use the sys.getsizeof function defined in the sys module.

sys.getsizeof(object[, default]):

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.

The default argument allows to define a value which will be returned if the object type does not provide means to retrieve the size and would cause a TypeError.

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

Usage example, in python 3.0:

>>> import sys
>>> x = 2
>>> sys.getsizeof(x)
24
>>> sys.getsizeof(sys.getsizeof)
32
>>> sys.getsizeof('this')
38
>>> sys.getsizeof('this also')
48

If you are in python < 2.6 and don't have sys.getsizeof you can use this extensive module instead. Never used it though.