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pythonpython-3.3function-objectdynamic-function

create dynamic functions keeping state

Python beginner question. Say you want to dynamically create a function that keeps some state (in the code below an integer i). Then as the function defined is an object, we could use it later on. In the following code, I add the functions to a list, and each call to print(fn(0)) should result in 0 1 2 3 4, but instead I get 4 4 4 4 4 as if only the latest value of i is used.

fns = []
for i in range(5):

    def fn(x):
        return x + i

    fns += [fn]
print(fns)

for fn in fns:
    print(fn(0))
quit()

Is it possible to accomplish in Python what this code tries to do?

Solution

  • To work around the late binding issue, you can use default keyword parameter:

    for i in range(5):
    def fn(x, i=i):  # <-----
        return x + i
    ....