Python declare variables only the first time I call a function [duplicate]

Question

This question already has answers here:

What is the Python equivalent of static variables inside a function? (31 answers)

Closed 9 years ago.

I have a function in python and I want to declare 2 variables only the first time I call this function and then change their values, something like this :

def function():
 x=0
 z=2
 if(x>z):
 other_function()
 else:
 x+=1

With this way,every time I call the function(), x becomes 0 and z becomes 2.

I tried to make them global outside of the function() but it gives me error :

UnboundLocalError: local variable 'x' referenced before assignment

How can I declare the values the first time I call the function()?

Answer 1

I disagree with the other answers, that only try to give a direct answer to your question.

You have a combination of state (the variables) with a function / functions using that state. That's what a class is for.

class myclass:
 def __init__(self):
 self.x = 0
 self.z = 2
 def myfunction(self):
 if self.x > self.z:
 other_function() # Possibly self.other_function()
 # if that one also uses x or z
 else:
 self.x += 1

Use as:

instance = myclass()
instance.myfunction()
instance.myfunction() # Etc

• adrian: that worked!thank you!

Answer 2

Welcome to closure

You should write some like this:

 
def f(x):
 def g(y):
 return x + y
 return g

def h(x):
 return lambda y: x + y
a = f(1)
b = h(1)
f(1)(5)
h(1)(5)

Answer 3

You can make use of lexical scope and global in Python:

x = 0
z = 2
def foo():
 global x
 if (x > z):
 bar()
 else:
 x += 1
foo()

Answer 4

You could use default function arguments

i.e.)

def function(x = None, z = None):
 if x is None:
 x=0
 if z is None:
 z=2

If you pass the value of x or z the passed value is used else it is initialized to the value given in the function

Answer 5

If you want to get x and z from somewhere outside of the function then they'll have to be passed to function as arguments. For example:

def function(a,b):
 if(a>b):
 other_function()
 else:
 a+=1
 return a, b

which can then be called with

x, z = function(0,2)

• Rafał Łużyński: this will not work unfortunately because a will be local, so a+=1 won't work. Yet your thinking is good.
• MarkHarley: ah, quite right. I'll edit to reflect

Answer 6

I would put them in a dictionary for better readability:

_VALS = {'x': 0, 'z': 2}
def function():
 if _VALS['x'] > _VALS['z']:
 other_function()
 else:
 _VALS['x'] += 1

Answer 7

You can abuse the mighty mutable default value with caution.

def func(vals = {'x': 0, 'z': 2}):
 print vals['x']
 vals['x'] += 1
func()
>> 0
func()
>> 1
func()
>> 2

Although I guess the proper way of doing this will be using a decorator:

def default_value_decorator(func):
 def inner(*args, **kwargs):
 returned_val = func(*args, **kwargs)
 inner.x += 1
 return returned_val
 inner.x = 0
 inner.z = 2
 return inner
@default_value_decorator
def func():
 print func.x
func()
>> 0
func()
>> 1

And a more reusable version where the starting values of x and z can be passed to the decorator:

def default_value_decorator(x, z):
 def a(func):
 def inner(*args, **kwargs):
 returned_val = func(*args, **kwargs)
 inner.x += 1
 return returned_val
 inner.x = x
 inner.z = z
 return inner
 return a
@default_value_decorator(0, 2)
def func():
 print func.x
func()
>> 0
func()
>> 1