Why Do My Functions in a Python Loop All Return the Same Value?

Understanding Late Binding when Creating Functions in Loops or Comprehensions

In an attempt to create functions within a loop, you may encounter a situation where all the functions return the same value despite expecting different outputs. This behavior occurs due to late binding, where the value of a variable is resolved at the time of execution.

Python's Late Binding

In Python, functions and lambdas by default bind variables at the time of execution. This means that any variables used within them are looked up when the function is called. In your loop, the loop iterator i is resolved later, and by that time, it references the final value of i after the loop.

For example, in the following code snippet:

functions = []

for i in range(3):
    def f():
       return i

    functions.append(f)

All the functions in the functions list reference the same value of i, which is 2 after the loop finishes.

Enforcing Early Binding

To resolve this issue, you need to force early binding, which ensures that the value of i is bound at the function definition time. One way to do this is by using a default value for the argument, as seen below:

functions = []

for i in range(3):
    
    def f(i=i):
       return i
    
    functions.append(f)

By providing a default value for the i argument, early binding is enforced. The value of i that is passed into the f function is resolved when the function is defined, not when it is executed.

Using a Function Factory

Another approach is to use a function factory that creates functions with the desired bindings. This involves creating an additional function that takes the variable you want to bind and returns a nested function with the early bound variable:

def make_f(i):

    def f():
        return i
        
    return f

In your loop, you can then use f = make_f(i) to create functions with the appropriate early bindings.

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