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RecursionError: maximum recursion depth exceeded

What it means

A function kept calling itself until Python stopped it after a set number of nested calls. Almost always the function has no base case (a condition that returns without calling itself again), or its base case is never reached. The traceback shows the same line repeated many times.

1. No base case

This raises the error

Broken
def factorial(n):
return n * factorial(n - 1)
print(factorial(5))
Output
Traceback (most recent call last):
  File "<exec>", line 4, in <module>
  File "<exec>", line 2, in factorial
  File "<exec>", line 2, in factorial
  File "<exec>", line 2, in factorial
  [Previous line repeated 992 more times]
RecursionError: maximum recursion depth exceeded

The fix

Fixed
def factorial(n):
if n <= 1: # base case: stop here
return 1
return n * factorial(n - 1)
print(factorial(5))
Output
120

The base case returns a value without another call, so the chain of calls ends and the results multiply back up to 120.

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2. A base case that is never reached

This raises the error

Broken
def count_down(n):
if n == 0:
return 'Liftoff!'
return count_down(n - 2)
print(count_down(5))
Output
Traceback (most recent call last):
  File "<exec>", line 6, in <module>
  File "<exec>", line 4, in count_down
  File "<exec>", line 4, in count_down
  File "<exec>", line 4, in count_down
  [Previous line repeated 992 more times]
RecursionError: maximum recursion depth exceeded

The fix

Fixed
def count_down(n):
if n <= 0: # catches 0 and anything below it
return 'Liftoff!'
return count_down(n - 2)
print(count_down(5))
Output
Liftoff!

Stepping by 2 from 5 gives 3, 1, -1 and so on, skipping 0 forever. A condition like n <= 0 catches every way past the end.

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3. A property that reads itself

This raises the error

Broken
class Person:
def __init__(self, name):
self._name = name
@property
def name(self):
return self.name
p = Person('Ana')
print(p.name)
Output
Traceback (most recent call last):
  File "<exec>", line 10, in <module>
  File "<exec>", line 7, in name
  File "<exec>", line 7, in name
  File "<exec>", line 7, in name
  [Previous line repeated 992 more times]
RecursionError: maximum recursion depth exceeded

The fix

Fixed
class Person:
def __init__(self, name):
self._name = name
@property
def name(self):
return self._name # the stored value, not the property
p = Person('Ana')
print(p.name)
Output
Ana

Inside the property, self.name runs the same property again, forever. Reading the stored attribute self._name ends the loop.

Try fixing it yourself in the editor

When you see this error, check

  • Find the line repeated in the traceback: that is the call that never stops.
  • Make sure every recursive function has a base case that returns without calling itself.
  • Check that each call moves closer to the base case, and prefer <= to == when steps can jump past it.
  • For recursion that is correct but very deep, rewrite it with a loop instead of raising sys.setrecursionlimit().

Related errors

All Python errors · How to read a traceback · Step through your code