PyMOTW: sys, Part 3: Memory Management and Limits

By Doug Hellmann
October 25, 2009

Memory Management and Limits

sys includes several functions for understanding and controlling memory usage.

Reference Counts

Python helps you manage memory with garbage collection. An object is automatically marked to be collected when its reference count drops to zero. To examine the reference count of an existing object, use getrefcount().

import sys

one = []
print 'At start         :', sys.getrefcount(one)

two = one

print 'Second reference :', sys.getrefcount(one)

del two

print 'After del        :', sys.getrefcount(one)

Notice that the count is actually one higher than expected because there is a temporary reference to the object held by getrefcount() itself.

$ python
At start         : 2
Second reference : 3
After del        : 2

See also

Control the garbage collector via the functions exposed in gc.

Object Size

Knowing how many references an object has may help you figure out where you have a cycle or a leak in your memory, but it isn’t enough to determine what objects are consuming the most memory. For that, you also need to know how big objects are.

import sys

class OldStyle:

class NewStyle(object):

for obj in [ [], (), {}, 'c', 'string', 1, 2.3, 
             OldStyle, OldStyle(), NewStyle, NewStyle(),
    print '%10s : %s' % (type(obj).__name__, sys.getsizeof(obj))

The size is reported in bytes.

$ python
      list : 36
     tuple : 28
      dict : 140
       str : 25
       str : 30
       int : 12
     float : 16
  classobj : 48
  instance : 36
      type : 452
  NewStyle : 32

For a more accurate estimate of the space used by a class, you can provide a __sizeof__() method to compute the value by aggregating the sizes of attributes of an object.

import sys

class MyClass(object):
    def __init__(self):
        self.a = 'a'
        self.b = 'b'
    def __sizeof__(self):
        return object.__sizeof__(self) + \
            sum(sys.getsizeof(v) for v in self.__dict__.values())

my_inst = MyClass()
print sys.getsizeof(my_inst)
$ python


Allowing infinite recursion in a Python application may introduce a stack overflow in the interpreter itself, leading to a crash. To eliminate this situation, the interpreter lets you control the maximum recursion depth using setrecursionlimit() and getrecursionlimit().

import sys

print 'Initial limit:', sys.getrecursionlimit()


print 'Modified limit:', sys.getrecursionlimit()

def generate_recursion_error(i):
    print 'generate_recursion_error(%s)' % i

except RuntimeError, err:
    print 'Caught exception:', err

Once the recursion limit is reached, the interpreter raises a RuntimeError exception so your program has an opportunity to handle the situation.

$ python
Initial limit: 1000
Modified limit: 10
Caught exception: maximum recursion depth exceeded while getting the str of an object

Maximum Values

Along with the runtime configurable values, sys includes variables defining the maximum values for types that vary from system to system.

import sys

print 'maxint    :', sys.maxint
print 'maxsize   :', sys.maxsize
print 'maxunicode:', sys.maxunicode

maxint is the largest representable regular integer. maxsize is the maximum size of a list, dictionary, string, or other data structure dictated by the C interpreter’s size type. maxunicode is the largest integer Unicode point supported by the interpreter as currently configured.

$ python
maxint    : 2147483647
maxsize   : 2147483647
maxunicode: 65535

Floating Point Values

The structure float_info contains information about the floating point type representation used by the interpreter, based on the underlying system’s float implementation.

import sys

print 'Smallest difference (epsilon):', sys.float_info.epsilon
print 'Digits (dig)              :', sys.float_info.dig
print 'Mantissa digits (mant_dig):', sys.float_info.mant_dig
print 'Maximum (max):', sys.float_info.max
print 'Minimum (min):', sys.float_info.min
print 'Radix of exponents (radix):', sys.float_info.radix
print 'Maximum exponent for radix (max_exp):', sys.float_info.max_exp
print 'Minimum exponent for radix (min_exp):', sys.float_info.min_exp
print 'Maximum exponent for power of 10 (max_10_exp):', sys.float_info.max_10_exp
print 'Minimum exponent for power of 10 (min_10_exp):', sys.float_info.min_10_exp
print 'Rounding for addition (rounds):', sys.float_info.rounds


These values depend on the compiler and underlying system, so you may have different results. These examples were produced on OS X 10.5.8.

$ python
Smallest difference (epsilon): 2.22044604925e-16

Digits (dig)              : 15
Mantissa digits (mant_dig): 53

Maximum (max): 1.79769313486e+308
Minimum (min): 2.22507385851e-308

Radix of exponents (radix): 2

Maximum exponent for radix (max_exp): 1024
Minimum exponent for radix (min_exp): -1021

Maximum exponent for power of 10 (max_10_exp): 308
Minimum exponent for power of 10 (min_10_exp): -307

Rounding for addition (rounds): 1

See also

Your system’s float.h contains more details about these settings.


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