What is meant by “using the EAFP principle” in Python? Could you provide any examples?
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
exceptstatements. 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.
try: x = my_dict["key"] except KeyError: # handle missing key
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.
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.
I’ll try to explain it with another example.
Here we’re trying to access the file and print the contents in console.
We might want to check if we can access the file and if we can, we’ll open it and print the contents. If we can’t access the file we’ll hit the
else part. The reason that this is a race condition is because we first make an access-check. By the time we reach
with open(my_file) as f: maybe we can’t access it anymore due to some permission issues (for example another process gains an exclusive file lock). This code will likely throw an error and we won’t be able to catch that error because we thought that we could access the file.
import os my_file = "/path/to/my/file.txt" # Race condition if os.access(my_file, os.R_OK): with open(my_file) as f: print(f.read()) else: print("File can't be accessed")
In this example, we’re just trying to open the file and if we can’t open it, it’ll throw an
IOError. If we can, we’ll open the file and print the contents. So instead of asking something we’re trying to do it. If it works, great! If it doesn’t we catch the error and handle it.
# # No race condition try: f = open(my_file) except IOError as e: print("File can't be accessed") else: with f: print(f.read())