Python in 30 Days: Day 15
Python Error Types
When we write code, we commonly make a typo or some other common error. If our code fails to run, the Python interpreter will display a message containing feedback with information on where the problem occurs and the type of error. It will also sometimes give us suggestions on a possible fix. Understanding different types of errors in programming languages will help us to debug our code quickly, and it will also make us better at what we do.
Let us see the most common error types one by one. First, let us open our Python interactive shell. Go to your computer terminal and write ‘python’. The Python interactive shell will be opened.
SyntaxError
Example 1: Syntax Error
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print 'hello world'
File "<stdin>", line 1
print 'hello world'
^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print('hello world')?
>>>
As you can see we made a syntax error because we forgot to enclose the string with parenthesis and Python already suggests the solution. Let us fix it.
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print 'hello world'
File "<stdin>", line 1
print 'hello world'
^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print('hello world')?
>>> print('hello world')
hello world
>>>
The error was a SyntaxError. After the fix, our code was executed without a hitch. Let’s see more error types.
NameError
Example 1: NameError
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print(age)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'age' is not defined
>>>
As you can see from the message above, the name age is not defined. Yes, indeed, we did not define an age variable but we were trying to print it out as if we had declared it. Now, let’s fix this by declaring it and assigning with a value.
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print(age)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'age' is not defined
>>> age = 25
>>> print(age)
25
>>>
The type of error was a NameError. We debugged the error by defining the variable name.
IndexError
Example 1: IndexError
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> numbers = [1, 2, 3, 4, 5]
>>> numbers[5]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list index out of range
>>>
In the example above, Python raised an IndexError, because the list has only indexes from 0 to 4, so it was out of range.
ModuleNotFoundError
Example 1: ModuleNotFoundError
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import maths
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'maths'
>>>
In the example above, I added an extra s to math deliberately and ModuleNotFoundError was raised. Let’s fix it by removing the extras from math.
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import maths
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'maths'
>>> import math
>>>
We fixed it, so let’s use some of the functions from the math module.
AttributeError
Example 1: AttributeError
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import maths
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'maths'
>>> import math
>>> math.PI
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: module 'math' has no attribute 'PI'
>>>
As you can see, I made a mistake again! Instead of pi, I tried to call a PI function from the maths module. It raised an attribute error, which means, that the function does not exist in the module. Let’s fix it by changing from PI to pi.
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import maths
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'maths'
>>> import math
>>> math.PI
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: module 'math' has no attribute 'PI'
>>> math.pi
3.141592653589793
>>>
Now, when we call pi from the math module we get the result.
KeyError
Example 1: KeyError
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> users = {'name':'Asab', 'age':250, 'country':'Finland'}
>>> users['name']
'Asab'
>>> users['county']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'county'
>>>
As you can see, there was a typo in the key used to get the dictionary value. so, this is a key error and the fix is quite straightforward. Let’s do this!
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> user = {'name':'Asab', 'age':250, 'country':'Finland'}
>>> user['name']
'Asab'
>>> user['county']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'county'
>>> user['country']
'Finland'
>>>
We debugged the error, our code ran and we got the value.
TypeError
Example 1: TypeError
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 4 + '3'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'
>>>
In the example above, a TypeError is raised because we cannot add a number to a string. The first solution would be to convert the string to int or float. Another solution would be converting the number to a string (the result then would be ’43’). Let us follow the first fix.
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 4 + '3'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'
>>> 4 + int('3')
7
>>> 4 + float('3')
7.0
>>>
Error removed and we got the result we expected.
ImportError
Example 1: TypeError
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from math import power
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: cannot import name 'power' from 'math'
>>>
There is no function called power in the math module, it goes with a different name: pow. Let’s correct it:
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from math import power
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: cannot import name 'power' from 'math'
>>> from math import pow
>>> pow(2,3)
8.0
>>>
ValueError
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> int('12a')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '12a'
>>>
In this case, we cannot change the given string to a number, because of the ‘a’ letter in it.
ZeroDivisionError
tech@Tech:~$ python
Python 3.9.6 (default, Jun 28 2021, 15:26:21)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 1/0
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>>
We cannot divide a number by zero.
We have covered some of the Python error types, if you want to check more about it check the Python documentation about Python error types. If you are good at reading the error types then you will be able to fix your bugs fast and you will also become a better programmer.
Now do some exercises for your brain and your muscles.
Exercises: Python in 30 Days: Day 15 – Python Type Errors
- Open your Python interactive shell and try all the examples covered in this section.
