Python Error Explainer

Pick the error in your traceback

Each explainer below is a focused deep dive into one exception: what the message literally means, a real runnable example with its full traceback, the specific causes, a debugging routine, and free PyDebug problems where you repair that exact error in the browser.

Fix the exact message in your traceback

These pages take the wording you actually see — the full message, quotes and all — and decode that one message end to end: what it means, a runnable example that raises it, the causes, the fix, and free practice where you repair that exact bug in the browser.

• 'int' object is not subscriptable — the value in front of your brackets is a number, not a container.
• 'str' object does not support item assignment — strings are immutable — rebuild, replace, or go through a list.
• can only concatenate str (not "int") to str — str() at the join, f-strings for sanity, and the input() trap.
• unsupported operand type(s) for +: 'int' and 'str' — the same mixed-types bug when the number is on the left.
• list indices must be integers or slices, not str — a key used on a list — the JSON shape confusion decoded.
• 'int' object is not iterable — for i in 5 needs a range(), and the count-vs-collection mix-up.
• unhashable type: 'list' — lists cannot be dict keys — the tuple fix and what hashable means.
• ValueError: invalid literal for int() with base 10 — what int() refuses, and the try/except that makes conversion safe.
• too many values to unpack (expected 2) — the star operator, maxsplit, and the split() habits that prevent it.
• IndexError: list index out of range — why len(list) is never a valid index, and the empty-list bug behind it.
• 'NoneType' object has no attribute — an in-place method or a missing return handed you None.
• ModuleNotFoundError: No module named — not installed, misspelled, wrong environment, or a shadowing file.
• UnboundLocalError: local variable referenced before assignment — assigning anywhere in a function makes the name local for the whole function.
• string indices must be integers — a dict key used on a string: the dict-loop and json.loads mistakes.
• 'NoneType' object is not subscriptable — sort() and append() return None, and so does a function with no return.
• can't multiply sequence by non-int of type 'str' — input() returns text, so '5' * '3' is not 15.
• 'int' object is not callable — sum = 0 overwrote the built-in, and the missing-operator variant.
• not enough values to unpack (expected 3, got 1) — split() with a separator that is not in the string.
• IndentationError: unexpected indent — stray leading spaces, tabs mixed with spaces, and pasted code.
• FileNotFoundError: No such file or directory — the working-directory trap, hidden extensions, and the pathlib fix.
• ImportError: cannot import name — the module was found, the name was not: circular imports and shadowing files.
• RecursionError: maximum recursion depth exceeded — the missing base case, and when raising the limit is the wrong answer.
• unterminated string literal — the missing quote, the trailing backslash, and multi-line strings done right.
• invalid character in identifier — smart quotes and lookalike Unicode from Word, PDFs and chat apps.
• can only concatenate list (not "str") to list — you meant append, not +.
• object of type 'int' has no len() — numbers have no length, and the len(str(n)) digit count.
• not all arguments converted during string formatting — % is also modulo, and the f-string fix.
• module has no attribute — lazy submodules, circular imports and shadowing filenames.
• UnicodeDecodeError: 'utf-8' codec can't decode byte — binary files, the wrong encoding, and the BOM.
• NameError: name 'true' is not defined — true/false/null are JavaScript and JSON, not Python.
• TypeError: 'NoneType' object is not iterable — the four ways None reaches a for loop, and the guard for each.
• SyntaxError: unexpected EOF while parsing — EOF means the parser hit the end inside an unclosed bracket, string or block.
• ValueError: math domain error — which math functions reject which inputs, and the cmath alternative.
• AttributeError: 'str' object has no attribute 'append' — strings are immutable, and join is the correct way to build one.
• ValueError: could not convert string to float — currency symbols, thousands commas, empty cells and the cleaning pipeline.
• IndexError: string index out of range — the valid index range, and why slicing never raises but indexing does.
• IndentationError: expected an indented block — the block header has a colon but no body — and the pass fix.
• TypeError: '<' not supported between instances of 'str' and 'int' — comparing text with numbers, and the sorting case that hides it.
• ValueError: time data does not match format — strptime directives, the ISO 8601 shortcut and timezone traps.
• UnicodeEncodeError: can't encode character — why utf-8 almost never fails, and the cp1252 and ascii cases.
• TypeError: cannot unpack non-iterable NoneType object — a function returned None instead of a tuple — find which return path.
• RuntimeError: dictionary changed size during iteration — deleting keys inside a loop, and the four safe patterns.

How to read any Python traceback

Read it bottom-up: the last line names the exception and its message; the lines above show the call chain that led there, with the deepest (most useful) frame last. Two facts direct everything else: the exception type tells you which rule was broken, and the message payload — the quoted name, key, index, or value — tells you which piece of your data broke it.

Every message above is also collected, with the rarer exceptions, in the complete Python errors guide. For the searching-and-narrowing method itself, see the Python debugging guide.

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