Common Python errors

what is written and what it means

What it is

Real Python 3.13 messages — all obtained by running, not from memory. Read them from the bottom up: the last line says what happened, the lines above say where (article 2.12).

Names and types

NameError: name 'cuty' is not defined The name doesn't exist. Usually a typo, or a variable declared further down the file than it is used.

AttributeError: 'str' object has no attribute 'push' The object has no such method. A frequent sign that the AI mixed up languages: push belongs to arrays in JavaScript; Python lists have append.

TypeError: can only concatenate str (not "int") to str A string was added to a number. Almost always the value came from an environment variable or from input and stayed a string (article 13.4).

TypeError: 'NoneType' object is not subscriptable A key or index was used on None. Which means a function above returned None instead of data — and that wasn't checked.

TypeError: f() missing 1 required positional argument: 'b' Called with too few arguments. Often after a signature was edited and not every call site was updated.

UnboundLocalError: cannot access local variable 'n' where it is not associated with a value Inside a function, a value was assigned to a variable declared outside. Python treats it as local. The cure is passing it as an argument, not global.

Data

KeyError: 'temp' The dictionary has no such key. The commonest error when parsing API responses: the key is there when all is well and disappears when the server returns an error.

IndexError: list index out of range An index past the end of the list. Check the list isn't empty — and remember the last element is [-1], not [len(xs)].

ValueError: invalid literal for int() with base 10: '-2.5' The string doesn't turn into a number. int() doesn't understand decimal notation — you want float() and then rounding.

ZeroDivisionError: division by zero Division by zero. Usually the denominator is the length of an empty list or a counter that hasn't had time to grow.

json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0) Something that isn't JSON was parsed as JSON: an empty string, an HTML error page, a truncated file. What to print is the raw response text, not the parsed data.

UnicodeDecodeError: 'utf-8' codec can't decode byte 0xd8 in position 96 A binary file was opened as text, or text in another encoding. For binary, the "rb" mode; for text, an explicit encoding=.

Files and modules

FileNotFoundError: [Errno 2] No such file or directory: 'cities.json' The file isn't there where the program was started from. A relative path is counted from the working directory, not from the file with the code (article 12.1).

ModuleNotFoundError: No module named 'rekvests' The library isn't installed, or the name is misspelled. If the name looks strange, it may not exist at all (article 4.8).

ImportError: cannot import name 'x' from partially initialized module 'y' (most likely due to a circular import) Two modules import each other. That isn't a Python error but a signal of mixed-up layers (article 13.1).

Syntax and execution

IndentationError: expected an indented block after function definition on line 1 The indentation was lost. Usually when pasting code in pieces.

SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='? One equals sign instead of two in a condition. Python says so right in the error text.

RecursionError: maximum recursion depth exceeded A function calls itself with no stopping condition. The traceback shows [Previous line repeated 996 more times].

sqlite3.IntegrityError: UNIQUE constraint failed: cities.name A table rule was broken: a record with that value already exists. That isn't a breakage but a guard doing its job — catch it and return a clear response (article 14.2).