Question

When writing concurrent/multithreaded code in Python, is it especially important to follow the ‘Easier to Ask for Forgiveness than Permission’ (EAFP) idiom, rather than ‘Look Before You Leap’ (LBYL)? Python’s exceptionally dynamic nature means that almost anything (e.g., attribute removal) can happen between looking and leaping—if so, what’s the point? For example, consider

# LBYL if hasattr(foo, 'bar'):     baz = foo.bar 

versus

# EAFP try:     baz = foo.bar except AttributeError:     pass 

In the LBYL example, the bar attribute could disappear from foo before the actual call to foo.bar is made, so do you gain anything from the check? If there’s a risk the attribute might disappear, you need locks and/or try/except clauses anyway.

One possible argument here is that this example makes the extremely pessimistic assumption that ‘antagonistic code’ is running that could yank the rug from under you at any moment. In most use cases, this is highly unlikely.

Answer 1

Your thoughts are correct. Some additional points:

If the attribute exists most of the time, try:except: might be much faster than the LBYL idiom.

If you don’t like the try: except: syntax, you can also write:

item = getattr(foo, 'bar', None) if item is None:     .... else:     ....