What is the EAFP principle in Python?

Question

What is meant by "using the EAFP principle" in Python? Could you provide any examples?

Answer 1

From the glossary:

Easier to ask for forgiveness than permission. This common Python coding style assumes the existence of valid keys or attributes and catches exceptions if the assumption proves false. This clean and fast style is characterized by the presence of many try and except statements. The technique contrasts with the LBYL style common to many other languages such as C.

An example would be an attempt to access a dictionary key.

EAFP:

try:
    x = my_dict["key"]
except KeyError:
    # handle missing key

LBYL:

if "key" in my_dict:
    x = my_dict["key"]
else:
    # handle missing key

The LBYL version has to search the key inside the dictionary twice, and might also be considered slightly less readable.

Answer 2

I call it "optimistic programming". The idea is that most times people will do the right thing, and errors should be few. So code first for the "right thing" to happen, and then catch the errors if they don't.

My feeling is that if a user is going to be making mistakes, they should be the one to suffer the time consequences. People who use the tool the right way are sped through.