Facing up to mistakes
INPUT · Slides
01 / 07
Run your code, get a wall of unfamiliar text back, and it can be unnerving. But nobody is telling you off — it is a report from Python.
The important part of the report is the last line. It comes in the shape kind: body, as in NameError: name 'nane' is not defined.
In this lesson, learn the names of the kinds and nothing else. Once a name alone is enough to make you think "ah, that one", the time you spend stuck drops sharply.
02 / 07
This is what you get when you use a name Python does not know. Nearly always it is a typo, or a variable used before it was made.
In the example below the name was made as name and then called nane. Python is honest about it: "nobody ever mentioned a nane to me".
is not defined means exactly that. Suspect your spelling, and the order you did things in.
name = "Yui"print(nane)Result
NameError: name 'nane' is not defined. Did you mean: 'name'?
03 / 07
You get this when you combine things of kinds that do not go together. Joining text to a number with + is the classic.
Python takes the view that the only thing you can add to text is more text, so 300 will not go on the end as it stands. str(300) turns it into text, and an f-string does the same job.
print("price: " + 300)Result
TypeError: can only concatenate str (not "int") to str
04 / 07
Reach for a position a list has not got and you get this. With three items the positions you can use are 0, 1 and 2. There is no 3.
list index out of range means "that position is past the end". Remember it as up to one less than len().
fruits = ["apple", "orange", "grape"]print(fruits[3])Result
IndexError: list index out of range
05 / 07
Ask a dictionary for a key that is not in it and you get this. The body carries the key you asked for, so you can see at once which one was wrong.
A list gives you IndexError, a dictionary gives you KeyError. The kind of container changes the name of the error — a neat detail.
prices = {"apple": 120, "orange": 80}print(prices["grape"])Result
KeyError: 'grape'
06 / 07
The kind is fine, but the contents are not in a workable shape. int("abc") is the standard example.
int() turns text into a number. Handing it text is entirely correct — but "abc" is not shaped like a number, so you are told that this particular value will not do.
TypeError is the wrong kind; ValueError is the right kind with the wrong contents. That is how you tell them apart.
print(int("abc"))Result
ValueError: invalid literal for int() with base 10: 'abc'
07 / 07
There is one more worth meeting: ZeroDivisionError, which turns up when you divide by zero. Working out a share per head when the head count is zero is the usual way in.
Here are the names against their causes.
| Name | What happened |
|---|---|
NameError | you used a name it does not know |
TypeError | you combined kinds that do not go together |
IndexError | you reached past the end of a list |
KeyError | you asked a dictionary for a key it has not got |
ValueError | the kind was right, the contents were not |
ZeroDivisionError | you divided by zero |
Those six account for most of what a beginner produces. Next comes the wall of text that arrives alongside this one line, and how to read it.
print(10 / 0)Result
ZeroDivisionError: division by zero