My question is regarding the rules of enclosing scope. In the snippet below if x=x is not passed to the header of F2 there will be an error
def f1():
x = 88
def f2(x=x): # Remember enclosing scope X with defaults
print(x)
f2()
f1()
however I dont understand why "x=x" is not needed in the lambda in the below snippet
def func():
x = 4
action = (lambda n: x ** n) # x remembered from enclosing def
return action
x = func()
print(x(2))
In the snippet below if x=x is not passed to the header of F2 there will be an error
No there won't:
>>> def f1():
... x = 88
... def f2():
... print(x)
... f2()
...
>>> f1()
88
You only need the default parameter value hack when you're trying to pass a value that might get changed later. f2 as I've written it captures the variable x from the enclosing scope, so it will print whatever x happens to be at the time. As you've written it, it captures the current value of the variable x, not the variable itself.
For example:
>>> def f1():
... x = 88
... def f2():
... print(x)
... x = 44
... f2()
...
>>> f1()
44
>>> def f1():
... x = 88
... def f2(x=x):
... print(x)
... x = 44
... f2()
...
>>> f1()
88
For a very common real-life example of where this difference is important, see Why do lambdas defined in a loop with different values all return the same result? in the official FAQ.
