Python errors, explained

You tried to loop over a value that is None, or passed it to a function that loops over it, like list(), sorted() or sum().

You used square brackets (x[0], x["name"], x[1:3]) on a value that is None.

You put () after something that isn't a function.

You gave a float (a number like 3.0 or 2.5) to something that only works with whole numbers, and Python refused to round it for you.

You used square brackets (x[0], x["key"], x[1:3]) on a value that is an integer.

You tried to mutate a string in place with s[0] = "X".

json.dumps (or json.dump) found a value it doesn't know how to write as JSON.

You called a function with name=value, and the function has no parameter called name.

You indexed a list with a string, like users["name"].

You called a function with fewer arguments than its def line asks for.

len() asks an object how many items it holds.

A function received more positional arguments than its def line accepts.

You tried to use a list as a dictionary key or as a member of a set.

You tried to add a number to None.

Python refuses to join a number and a string with +.

Your code calls df.append(...), and that method no longer exists.

You called a method or accessed an attribute on None.

You used dot access (user.name) on a dictionary.

You called a method that lists don't have.

You asked a string for a method it doesn't have.

You imported a module successfully, but the function or class you tried to use isn't inside it.

Something asked a pandas Series for a single True or False, and a Series of many values can't give one.

You passed int() a string that isn't a valid base-10 integer.

You asked Python to unpack an iterable into more variables than the number of items it contains.

You tried to unpack an iterable into a fixed number of variables, but the iterable had more items than variables.

The file has an import with a leading dot (from .totals import invoice_total), and Python doesn't know which package that dot refers to.

Python found and successfully imported module Y, but when it tried to bind the name X from inside Y, that name was not there.

Two modules import each other.

Python opened a block with : (after if, for, def, class, while, try, else, with, etc.) but found no indented lines underneath it.

A line starts further to the right than Python expected.

Python's indentation is part of its grammar.

Something asked the asyncio event loop to do work after the loop had already shut down.

You called asyncio.run() (or loop.run_until_complete()) from code that is already running inside an event loop.

You are iterating over a dict (or a set, which raises the same kind of error) and you added or removed a key during the loop.

You opened a bracket ((, [ or {) and Python never found the one that closes it.

Python could not parse your source code.

Python found an opening quote and reached the end of the line without finding the matching closing quote.

You asked Python for an item at a position that doesn't exist in the list.

You indexed into a string at a position that doesn't exist.

Your process hit the operating system's limit on open file descriptors.

You tried to bind a server to a port that something else already holds.

Windows looked for a program called python in every folder listed in your PATH environment variable and didn't find one.

Your code reached the machine, and the machine said no.

The OS tried to open the path you gave it, and there is nothing at that path.

A KeyError is raised when you try to access a key in a dictionary (or a dict-like object) that does not exist.

Python asked the operating system for memory and was refused.

Python searched every folder in sys.path (your script's directory, the standard library, site-packages) and could not find a package or module called X.

Python tried to look up the name X and couldn't find it in any scope: not in the local scope, not in the global scope, and not in the built-ins.

The path exists, but the OS refused the operation because your process does not have permission for it.

Every function call adds a frame to Python's call stack.

The connection was made and the server presented a certificate, but Python could not build a chain of trust from that certificate up to a root authority it recognises.

Python uses indentation to mark where blocks start and end.

You sent a request, and no answer came back within the time you allowed.

Python decides at compile time, not run time, which names inside a function are local.

You opened a file (or decoded a bytes object) telling Python it was UTF-8, but the bytes don't form valid UTF-8.

You are writing text out (to a file, a terminal, a socket), and the encoding in use cannot represent one of the characters.

You tried to divide a number by zero.

pip refused to install into the Python that came with your operating system.

json.loads or json.load could not parse the input as JSON.

DNS could not turn the hostname into an IP address.