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try-except-in-loop (PERF203)

Derived from the Perflint linter.

What it does

Checks for uses of except handling via try-except within for and while loops.

Why is this bad?

Exception handling via try-except blocks incurs some performance overhead, regardless of whether an exception is raised.

To optimize your code, two techniques are possible:

  1. Refactor your code to put the entire loop into the try-except block, rather than wrapping each iteration in a separate try-except block.
  2. Use "Look Before You Leap" idioms that attempt to avoid exceptions being raised in the first place, avoiding the need to use try-except blocks in the first place.

This rule is only enforced for Python versions prior to 3.11, which introduced "zero-cost" exception handling. However, note that even on Python 3.11 and newer, refactoring your code to avoid exception handling in tight loops can provide a significant speedup in some cases, as zero-cost exception handling is only zero-cost in the "happy path" where no exception is raised in the try-except block.

As with all perflint rules, this is only intended as a micro-optimization. In many cases, it will have a negligible impact on performance.

Example

string_numbers: list[str] = ["1", "2", "three", "4", "5"]  # `try`/`except` that could be moved out of the loop: int_numbers: list[int] = [] for num in string_numbers:  try:  int_numbers.append(int(num))  except ValueError as e:  print(f"Couldn't convert to integer: {e}")  break  # `try`/`except` used when "look before you leap" idioms could be used: number_names: dict[int, str] = {1: "one", 3: "three", 4: "four"} for number in range(5):  try:  name = number_names[number]  except KeyError:  continue  else:  print(f"The name of {number} is {name}")

Use instead:

string_numbers: list[str] = ["1", "2", "three", "4", "5"]  int_numbers: list[int] = [] try:  for num in string_numbers:  int_numbers.append(int(num)) except ValueError as e:  print(f"Couldn't convert to integer: {e}")  number_names: dict[int, str] = {1: "one", 3: "three", 4: "four"} for number in range(5):  name = number_names.get(number)  if name is not None:  print(f"The name of {number} is {name}")

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