Stop iterative function python that keeps iterating

I have a function below, that takes a value as an input, and determines a new_value (the new_value will always be less than the original_value). If the difference between the new_value and the original_value is less than a specified tolerance, the new_value is returned. If the tolerance is greater, the function is called again with the new_value as the argument, to calculate a new_new_value. Depending on the original_value entered, sometimes the tolerance condition is never met. How can I make the function stop iterating after a certain number of iterations? I have tried to put in count=0, the count+=0 after the else, but that doesn't work as it returns to zero each time the function is called again. Thanks

def calc_value(original_value):
   # equations to calculate new_value
   if original_value - new_value < tolerance:
      return new_value
   else calc_value(new_value)

Solution

If you want to stop iterating after you reach a certain iteration depth you will need to pass a counter into the function. This can be done with a default argument in the function:

def calc_value(original_value, count = 0):
   # equations to calculate new_value
   if original_value - new_value < tolerance:
      return new_value
   else:
      if count > 100:
          return new_value  # Handle reaching limit
      calc_value(new_value, count + 1)

Every time you call the function reccursively, add 1 to the count and check if the limit has been reached.

By using a default argument, you don't need to bother setting it when you call the function:

calc_value(42)

You could even take this further and add a limit argument to specify the iteration limit:

def calc_value(original_value, count = 0, limit = 100):
    # ...
    if count > limit:
        return new_value
    calc_value(new_value, count, limit)

Then you can define the arguments either by order, or by specifying the argument:

# Default values: count = 0, limit = 100
calc_value(42)

# Will set count to default 0, and limit to 150
calc_value(42, limit=150)

# Will set count to 5, and limit to 200
calc_value(42, 5, 200)

# Will set count to 5, and limit to 150
calc_value(42, limit=150, count=5)