A closure is a function that captures variables from the scope where it was defined — not just their values at the moment of creation, but the variables themselves. When the function is called later, it reads the variable as it exists at call time, not at definition time.
Terms used here (standalone):
- Scope: the region of code where a name is visible. In Python, a function body has its own local scope; the enclosing function’s variables are in the enclosing scope.
- Closure: a function bundled with the environment (set of captured variables) from its defining scope. The function “closes over” those variables.
- Late binding: the value of a captured variable is looked up when the function is called, not when it is defined.
Consider this code:
fns = []
for i in range(3):
fns.append(lambda x: x + i)
results = [f(10) for f in fns]
Trace it carefully:
range(3)produces0, 1, 2.- Three lambda functions are created, each capturing the variable
i. - After the loop finishes, what is the value of
i? - All three lambdas are called with
x=10. What do they each return?
Think about when i is looked up — at lambda-creation time, or at call time.