lambda functions and list of functions in Python

Question

I have an array of functions, for example:

>>> def f():
...     print "f"
... 
>>> def g():
...     print "g"
... 
>>> c=[f,g]

Then i try to create two lambda functions:

>>> i=0
>>> x=lambda: c[i]()
>>> i+=1
>>> y=lambda: c[i]()

And then, call them:

>>> x()
g
>>> y()
g

Why c[i] in lambda are the same?

Answer 1

That's because the lambda function is fetching the value of the global variable i at runtime:

>>> i = 0
>>> x=lambda z = i : c[z]() #assign the current value of `i` to a local variable inside lambda
>>> i+=1
>>> y =lambda z = i : c[z]()
>>> x()
f
>>> y()
g

A must read: What do (lambda) function closures capture?

Answer 2

In Python closures don't capture actual values, but instead they capture namespaces. So when you use i inside your function it's actually looked up in the enclosing scope. And the value there has already changed.

You don't need all those lambda s and lists to see this.

>>> x = 1
>>> def f():
...   print(x)
...
>>> x = 2
>>> def g():
...   print(x)
...
>>> g()
2
>>> f()
2