Understanding recursion in Python

lets walk through the execution.

fact(5):
   5 is not 0, so fact(5) = 5 * fact(4)
   what is fact(4)?
fact(4):
   4 is not 0, so fact(4) = 4 * fact(3)
   what is fact(3)?
fact(3):
   3 is not 0, so fact(3) = 3 * fact(2)
   what is fact(2)?
fact(2):
   2 is not 0, so fact(2) = 2 * fact(1)
   what is fact(1)?
fact(1):
   1 is not 0, so fact(1) = 1 * fact(0)
   what is fact(0)?
fact(0):
   0 IS 0, so fact(0) is 1

Now lets gather our result.

fact(5) = 5* fact(4)

substitute in our result for fact(4)

fact(5) = 5 * 4 * fact(3)

substitute in our result for fact(3)

fact(5) = 5 * 4 * 3 * fact(2)

substitute in our result for fact(2)

fact(5) = 5 * 4 * 3 * 2 * fact(1)

substitute in our result for fact(1)

fact(5) = 5 * 4 * 3 * 2 * 1 * fact(0)

substitute in our result for fact(0)

fact(5) = 5 * 4 * 3 * 2 * 1 * 1 = 120

And there you have it. Recursion is the process of breaking a larger problem down by looking at it as successfully smaller problems until you reach a trivial (or "base") case.

Break the problem down into its execution steps.

fact(5)
| 5  * fact(4)
|| 5 * (4 * fact(3))
||| 5 * (4 * (3 * fact(2))
|||| 5 * (4 * (3 * (2 * fact(1))))
||||| 5 * (4 * (3 * (2 * (1 * fact(0)))))
|||||| 5 * 4 * 3 * 2 * 1 * 1
120

Your function simply calls itself, just as any other function can call it. In this case however, your function needs a stopping point so that it doesn't infinitely recurse (causing a Stack Overflow!). In your case this is when n is 0 (it should probably be 1 instead).

Keep in mind that each invocation of fact(), whether invoked externally or invoked by itself, gets its own distinct set of local variables.

fact(1) has n of 5
   fact(2) has n of 4
      fact(3) has n of 3
         fact(4) has n of 2
            fact(5) has n on 1
               fact(6) has n of 0

The deepest ones (here, fact(6) is deepest) are computed completely before the levels above them in the callstack are able to finish.

So

• fact(6) returns a 1 to fact(5) (termination case).
• fact(5) returns a 1 to fact(4) (1*1)
• fact(4) returns a 2 to fact(3) (2*1)
• fact(3) returns a 6 to fact(2) (3*2)
• fact(2) returns a 24 to fact(1) (4*6)
• and finally fact(1) returns 120 (5*24) to its caller, whatever that may be.