Errors (also called Exceptions)


When something goes wrong, Python responds by giving you a rude message that might look something like this:

Traceback (innermost last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 1, in f
TypeError: illegal argument type for built-in operation

You can usually ignore everything except the last line, although the earlier lines may – if you look at them closely – give some hints about where the trouble happened.

This sheet is a brief guide to some of the commoner things you might see on that last line, what they mean and what you might have done to provoke them.

Error messages

Attribute Errors, Key Errors, Index Errors

These have messages starting AttributeError: , KeyError: or IndexError: . They all mean something rather similar: you were trying to get at a part of an object (one element of a list, for instance), but you asked for a non-existent part of the object. So, if x is the list [1, 2, 3] and you ask for x[100] you’ll get an IndexError.

Name Errors

These all have messages starting NameError: . They mean I’ve never heard of this thing . The thing Python’s never heard of is what comes after NameError: . This might mean that you mistyped something: primt x instead of print x, say. It might also mean that you used a variable (i.e., a name) before it was defined. Or that you used a function before it was defed.

One way this can sometimes happen is if you forget the quotation marks around a string.

Syntax Errors

These all have messages starting SyntaxError: . What they have in common is that you said something Python couldn’t even begin to make sense of. If someone says I apple eat like the to then that’s a syntax error!

SyntaxError: invalid syntax

This can mean lots of things. Here are some examples of things that provoke it.

+ + +          # Add  *what* to  *what*?
1z6            # Is it a number? Is it a variable? Is it a mistake?
for            # After  ``for`` there ought to be some more stuff
while if:      #  ``if`` isn't a thing that can be true or false!
def 1(x):      #  ``1`` is a number. You can't use it as a name for a function
f([)           # The  ``[`` starts a list, but you never finished it
if 1==2        # You missed out the : at the end of the line
if x=y:        # That should say  ``==``, not  ``=``

One way in which you can get slightly surprising syntax errors is if you mess up the indentation (spaces at starts of lines), so if you can’t find anything else wrong it’s worth checking that the indentation is right.

SyntaxError: invalid token

This usually means that you messed up when typing a string. Maybe you missed off the final quotation mark, or tried to put apostrophes into a string surrounded by single quotes, or something like that.

Type Errors

These all have messages starting TypeError: . What they have in common is that Python was expecting one kind of object — a number, maybe — and you gave it another — a string, or a list, perhaps.

A lot of these things can happen in non-obvious ways. For instance, if you hand something that isn’t a number to one of the graphics functions – move(), draw() etc – then you are likely to get a TypeError.

TypeError: illegal argument type for built-in operation

Means: You asked Python to do a built-in operation (i.e., something like +) on the wrong sort of object. For instance, you might have asked it to add a number to a string.

TypeError: len() of unsized object

You asked for the length of something that doesn’t have a length. len() makes sense for strings, lists and tuples (you don’t need to know what a tuple is), and not much else.

TypeError: not enough arguments; expected 1, got 0

You tried to use a function, but the function was defined with more arguments than you gave it. You probably forgot one!

TypeError: number coercion failed

You tried to do something using numbers, but one of the things involved wasn’t a number. You can get this by doing 4+'ouch'.

TypeError: too many arguments; expected 1, got 3

You tried to use a function, but the function was defined with fewer arguments than you gave it.

TypeError: unsubscriptable object

You tried to do something like a[3] where a was the wrong kind of thing. This can easily happen if you use a function that expects a string and give it something else instead.