Why does Python handle a KeyError differently than an IndexError (in this case)? -

i trying out various one-line solutions problem of defining variable if not exist , noticed python handles dicts , lists/tuples differently. these errors seem entirely parallel me, i'm confused why there discrepancy.

dictionary keyerror handling

existing_dict = {"spam": 1, "eggs": 2} existing_dict["foo"] = existing_dict["foo"] if not keyerror else 3 

returns {"spam": 1, "eggs": 2, "foo": 3}

notice i'm referencing non-existing key in both left , right hand sides; python has no problem handling keyerror in either clause appears.

list indexerror handling (also true tuples)

existing_list = ["spam","eggs"] existing_list[2] = existing_list[2] if not indexerror else ["foo"] 

returns indexerror: list assignment index out of range

it's not difficult @ around specific error (answer here), i'm curious why there difference in these cases. in both situations, there seems error in both assignee/assignment clauses 1 "if not" error catch.

in both cases, keyerror , indexerror just classes , both true:

>>> bool(keyerror) true >>> bool(indexerror) true 

all class objects test true in python, see truth value testing.

you cannot use conditional expressions test exception; both examples, else value picked always, assigned; tests equivalent to:

existing_dict["foo"] = 3 existing_list[2] = ["foo"] 

you'd use exception handling instead, or use length test.

the exception caused because assignment list indexes only works if index exists:

>>> empty = [] >>> empty[0] = none traceback (most recent call last):   file "<stdin>", line 1, in <module> indexerror: list assignment index out of range 

this difference in how dictionary , list works; lists can append to, grow number of indices. cannot dictionary (there no order), add new key-value pair need assign it. on other hand, if lists supported arbitrary index assignment, happen indices in between? if list empty assigned index 42; happens indices 0-41?

either catch exception try/except:

try:     existing_list[2] = "foo" except indexerror:     existing.append('foo') 

this replaces existing value @ index 2, or appends if index doesn't yet exist.

you try test length:

if len(existing_list) <= 3:     existing_list.append('foo') 

and it'll appended if there not yet @ least 3 elements.

for dictionaries, test key:

if 'foo' not in existing_dict:     existing_dict['foo'] = 3