Question

I recently came across the idea of defining a function within a function in Python. I have this code and it gives this error:

---

def f1(a):
    def f2(x):
       return a+x
    return 2*a

---

Error: On calling f2(5)

Traceback (most recent call last):
File "<pyshell#19>", line 1, in <module>
f2(5)
NameError: name 'f2' is not defined

---

I am having some difficulty understanding the way global variables are used across functions or even in recursive calls. I would really appreciate it if someone would point out my mistake and maybe help me along the way. Thanks!!

Answer 1

You defined f2 in the local namespace of f1 only; it is not available globally.

If you want such a nested function to be available at the module level, you’d have to either return it from the function, or define a global variable to store it in:

def f1(a):
    def f2(x):
       return a+x
    return 2*a, f2

then call that as result, f2 = f1(somevariable_or_literal).

The global approach is not recommendable (using a global rarely is) but would look something like:

f2 = None

def f1(a):
    global f2
    def f2_local(x):
         return a+x
    f2 = f2_local
    return 2*a

at which point f2 will be set when you have called f1.