Python errors, explained

You tried to loop over (or unpack, or *-spread) something that turned out to be None.

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 used square brackets — x[0], x["key"], x[1:3] — on a value that turned out to be an integer.

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

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

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

Python refuses to glue a number and a string together with +.

You called a method or accessed an attribute on None.

You called a method on a list that lists simply don't have.

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

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 inside code that an event loop was already running.

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

You asked Python to unpack an iterable into more variables than it actually contained.

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

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.

Python's indentation is part of its grammar.

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.

The import machinery walked every entry 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 locally, not globally, not in 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 could not parse your source code.

Python uses indentation to delimit blocks.

You asked, and nothing 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 has no way to 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.