Static member of a function in Python ? [duplicate]

Question

This question already has answers here:

Closed 13 years ago.

Possible Duplicate:
Static class variables in Python
What is the Python equivalent of static variables inside a function?

How can I use static fields in Python ?

for example i want to count how many times the function has been called - how can i do this ?

• Tadeck: @PaulManta: I disagree. Your link is not related to class static fields (as OP says), but to " the static member at the function level, as opposed to the class level ", as stated by OP within the question you gave use link to.
• Paul Manta: @Tadeck The word 'class' doesn't even appear in the question... But the phrase 'static member of a function' does.
• Tadeck: @PaulManta: Please read the question you linked to again - there are both words: " class " and " function ". In the current question (this question) there is a reference to " static fields ". Because of that, and because I have never heard of " static fields " in reference to static variables within function, but I have heard of " static fields " in reference to static member variables of a class, I assumed the current question is about static member variables of a class. Unless you will prove otherwise, I suppose the upvotes of my first comment mean some support in this matter.
• Paul Manta: @Tadeck Regarding votes: Good for you, here are two internet credits. Seriously, what's the argument about? The example is clear, he wants to know how to count how many times a function was called. In lack of a better term he used the rather incorrect 'static fields', but his intention is still clear from the example and title. Where's the problem?
• Paul Manta: @Tadeck The word 'class' appears in this context, 'how does one implement the static member at the function level, as opposed to the class level'. Lol at you making such a big deal out of this.

Answer 1

If you wish to count how many times a method has been called, no matter which instance called it, you could use a class member like this:

class Foo(object):
 calls=0 # <--- call is a class member
 def baz(self):
 Foo.calls+=1
foo=Foo()
bar=Foo()
for i in range(100): 
 foo.baz()
 bar.baz()
print('Foo.baz was called {n} times'.format(n=foo.calls))
# Foo.baz was called 200 times

---

When you define calls this way:

class Foo(object):
 calls=0 

Python places the key-value pair ('calls', 0) in Foo.__dict__.

It can be accessed with Foo.calls. Instances of Foo, such as foo=Foo(), can access it with foo.calls as well.

To assign new values to Foo.calls you must use Foo.calls = .... Instances can not use foo.calls = ... because that causes Python to place a new and different key-value pair in foo.__dict__, where instance members are kept.

Answer 2

Here's a decorator adding counting to a function.

import functools
def count_calls(func):
 @functools.wraps(func)
 def decor(*args, **kwargs):
 decor.count += 1
 return func(*args, **kwargs)
 decor.count = 0
 return decor

Usage:

>>> @count_calls
... def foo():
... pass
...
>>> foo.count
0
>>> foo()
>>> foo.count
1

• jfs: the decorator might be better called count_calls
• Patryk: It's quite the most complicated way of doing it, isn't it ?
• ShadowRanger: @Patryk: Not at all; defining a decorator means you can count calls on any number of functions by adding a single line prefixing the function definition. Using the class attribute to count calls to a method doesn't generalize like that (it could, but it would end up equivalent to the decorator solution, more verbose, and likely somewhat slower).

Answer 3

Here is some example counting the number of calls of all objects of the same class:

class Swallow():
 i = 0 # will be used for counting calls of fly()
 def fly(self):
 Swallow.i += 1

And this is the proof:

>>> a = Swallow()
>>> b = Swallow()
>>> a.fly()
>>> a.i
1
>>> Swallow.i
1
>>> b.fly()
>>> b.i
2
>>> Swallow.i
2

so you can read it by giving the object name or class name.

Answer 4

Here's one simplistic way to do it:

def func():
 if not hasattr(func, 'counter'):
 func.counter = 0
 func.counter += 1
 counter = 0 # Not the same as `func.counter`
 print(func.counter)

Or if you don't like the if being executed on every call, you can do:

def func():
 func.counter += 1
 print(func.counter)
func.counter = 0