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List lambda

Hi! def cm(): return [lambda x: i*x for i in range(3)] for m in cm(): print m(1) Output:222 And my question is about lambda. The expression in this [lambda x: i*x for i in range(3)] Work as lambda x: 2*x for i in range(3)] And return 222 I do not understand why that After all range(3)=0,1,2 but no 2,2,2 Why it returns 2,2,2?

2 Answers

New Answer

+15

If we unpack the list comprehension and lambda function, cm() rougly looks like def cm(): def f(x): return i*x funcs = [] for i in range(3): funcs.append(f) return funcs Now the issue seems clearer: All three functions in the list refer to the same variable i, which is not local to them. So when these fuctions are called, 2, the last value of i coming from iterating over range(3), is used. For more experiments, we can try def cm(): def f(x): return i*x funcs = [] for i in range(3): funcs.append(f) i = 10 return funcs This time output is three 10s, as 10 is the last value of i. A quick way to avoid this is to save i as a variable local to the lambdas: def cm(): return [lambda x, j=i: j*x for i in range(3)] A variant of this is also in the Python FAQ: https://docs.python.org/3/faq/programming.html#why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result

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Maybe this is what you want to do🤔 https://code.sololearn.com/cJ070QKCnhBy