Python Control Flow: EAFP Vs LBYL

[scarecrow1123]

EAFP - Easier to Ask for Forgiveness than Permission
LBYL - Look Before You Leap

These are two different ways to do control flow. LBYL style pertains to writing if/else blocks to make decisions. According to [1], in this not so standard Pythonic way of doing control flow, exceptional cases get the emphasis by the way the conditions are expressed. A common example as below:

if "key" in dict_:
  value += dict_["key"]

As [1] suggests, in simpler words, the above piece of code conveys the special case in an emphasized way rather than showing us what is normal. Using try/except blocks(EAFP), we write what is normal and handle exceptions that may rise out of it. This becomes easier to convey the more natural cases as follows:

try:
    value += dict_["key"]
except KeyError:
    pass

[2] adds another important case where EAFP helps to avoid race conditions. In a multi-threaded environment, in the above if/else block, assume a thread has passed the if condition. Before the next statement gets executed in the current thread, another thread may inadvertently remove the key from dict_ which would cause an exception in the original thread. However, in the try/except case, this cannot happen. Another way to solve this problem is to use standard locking mechanisms.

[1] - Idiomatic Python: EAFP versus LBYL

[2] - Chapter 15. Context Managers and else Blocks, Fluent Python, Luciano Ramalho