RecursionError: maximum recursion depth exceeded

A function is calling itself too many times. Python’s default limit is 1000 recursive calls.

Fix 1: Add or fix the base case

# ❌ No base case — infinite recursion
def factorial(n):
    return n * factorial(n - 1)

# ✅ Add a base case
def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n - 1)

Fix 2: Check for accidental recursion

# ❌ __str__ calls itself via string formatting
class User:
    def __str__(self):
        return f"User: {self}"  # Calls __str__ again!

# ✅ Reference the attribute, not the object
class User:
    def __str__(self):
        return f"User: {self.name}"

Fix 3: Convert to iteration

If the recursion is legitimate but deep, rewrite it as a loop:

# ❌ Recursive — fails for large lists
def flatten(lst):
    result = []
    for item in lst:
        if isinstance(item, list):
            result.extend(flatten(item))
        else:
            result.append(item)
    return result

# ✅ Iterative with a stack
def flatten(lst):
    stack, result = [lst], []
    while stack:
        current = stack.pop()
        if isinstance(current, list):
            stack.extend(current)
        else:
            result.append(current)
    return result

Fix 4: Increase the limit (last resort)

import sys
sys.setrecursionlimit(5000)

This is a band-aid. If you need more than 1000 levels, you probably need an iterative approach instead.

Debugging tip

Add a counter to see where the recursion goes wrong:

def my_func(n, depth=0):
    print(f"depth={depth}, n={n}")
    if depth > 20:
        raise Exception("Too deep — check your logic")
    return my_func(n - 1, depth + 1)

See also: Python cheat sheet | Maximum call stack exceeded fix (JavaScript equivalent)