lambda in python reference mind puzzle

Question

Consider this example

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def dump(value):
    print value

items = []    
for i in range(0, 2):
    items.append(lambda: dump(i))

for item in items:
    item()

output:

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1
1

how can i get:

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0
1

Answer 1

You can use a parameter with a default value on the lambda:

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for i in range(0, 2):
    items.append(lambda i=i: dump(i))

This works because the default value is evaluated when the function is defined, not when it is called.

Answer 2

You can use:

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for i in range(0, 2):
    items.append(lambda i=i: dump(i))

to capture the current value of i

This works because default parameter values are evaluated at function creation time, and i is not the external variable, but a function parameter that will have the value you want as default.

A more detailed explanation can be found in this answer.

Answer 3

For completeness, you can also do:

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for i in range(0, 2):
    items.append((lambda i: lambda: dump(i))(i))