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In this Python Tutorial chapter

0. Intro to Python Tutorial
1. History and Philosophy of Python
2. The Interpreter, an Interactive Shell
3. Execute a Script
4. Structuring with Indentation
5. Assignment Expressions
6. Data Types and Variables
7. Type Annotations
8. Operators
9. Sequential Data Types
10. List Manipulation
11. Shallow and Deep Copy
12. Dictionaries
13. Sets and Frozen Sets
14. Sets Examples
15. Keyboard Input
16. Conditional Statements
17. Structural Pattern Matching
18. Adventure Game with Structural Pattern Matching
19. While Loops
20. For Loops
21. Output with Print
22. Formatted Output
23. Working with Dictionaries and while Loops
24. Functions
25. Passing Arguments
26. Parameters And Arguments
27. Namespaces
28. Global vs. Local Variables and Namespaces
29. File Management
30. Modular Programming and Modules
31. Packages
32. Errors and Exception Handling

7. Type Annotations

By Melisa Atay. Last modified: 05 Jul 2022.

If you know another programming language such as C or C++, you are used to declaring what data type you are working with. For example, this is how you would declare an integer in C.

int a;

from this moment on, we know that "a" is of type integer. Then we would maybe assign an integer to a.

 a = 3 

However we don't do it like that in Python. Let's go through what we already know.

a = 3
print(type(a))

<class 'int'>

As a is assigned an integer, it belongs to the integer class itself, without us having to say

 a = int() 

beforehand. a's data type would change accordingly, when it is assigned values from other data types.

a = 'hello'
print(type(a))

<class 'str'>

a = 3.14
print(type(a))

<class 'float'>

If Python is capable of determining the types itself, why are even type annotations useful?

Type Annotations: Why

• You understand where the type errors stem from.
• If you are using an IDE, such as Visual Studio Code, typing the type annotations would allow you to access the built-in functions easier.

  [Built-in Function Hints]

• When a variable is of no type, you wouldn't be able to automatically access the built-in functions that you intend to use.

  [No hints]

• You would get syntax-highlighting as a warning before you even run your code.

• Your code will be more readable & understandable. This is especially useful if you are working together with others.

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Type Annotations: How

Example 1

In this example you see the ord()function, which takes a string as input and converts it into an integer by using the ASCII values of the letters it contains.

def my_function(a:int,b:str)->int:
    return a + ord(b)

print(my_function(3,'a'))

100

my_function() works perfectly, since b has to be of type string and it is of type string. Let's try it otherwise:

print(my_function(3,5))

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_19492/488431691.py in <module>
----> 1 print(my_function(3,5))

/tmp/ipykernel_19492/2046822703.py in my_function(a, b)
      1 def my_function(a:int,b:str)->int:
----> 2     return a + ord(b)

TypeError: ord() expected string of length 1, but int found

Here we receive a TypeError because we wrote an integer, and not a string for b. Since ord() needs a string to function, the code doesn't work.

print(my_function('a','a'))

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_19492/1936946746.py in <module>
----> 1 print(my_function('a','a'))

/tmp/ipykernel_19492/2046822703.py in my_function(a, b)
      1 def my_function(a:int,b:str)->int:
----> 2     return a + ord(b)

TypeError: can only concatenate str (not "int") to str

Here we receive another TypeError because we wrote a string, and not an integer for a. While b is sucessfully converted into an integer by the ord() function, it is yet not possible to concatenate strings and integers.

print(my_function(4.2,'a'))

101.2

Interestingly our function runs now, yet our argument a is of type float and not integer as we declared at the very beginning.

Example 2:

In this example, we are going to see how the functions can be given variables that can work without raising any errors. However, that can be still be very problematic.

def add_together(a:int,b:int)->int:
    return a + b 

def return_the_last_digit(a:int)->int:
    return a % 10

our_sum = add_together(38,57)
print(return_the_last_digit(our_sum))

5

our_sum = add_together('add',' together')

print(return_the_last_digit(our_sum))

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_19492/576833914.py in <module>
----> 1 print(return_the_last_digit(our_sum))

/tmp/ipykernel_19492/1809188113.py in return_the_last_digit(a)
      1 def return_the_last_digit(a:int)->int:
----> 2     return a % 10

TypeError: not all arguments converted during string formatting

How can we run a TypeError check before we actually run the program?

By installing mypy and running it before you run your code, you could avoid type errors. Mypy checks your annotations and gives a warning if a function is initialized with the wrong datatype.

[Mypy]

All in all, type annotations are very useful and it can save a lot of time for you and it can make your code readable for both yourself and the others.

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