Logical conditions in Python explained

Introduction to conditionals in Python

There are 5 skills you need to master in your tool set for building algorithms with Python:

1. Variables.
2. Conditionals.
3. Lists (or arrays).
4. Loops.
5. Functions.

The use of conditionals is the easiest way we have to make decisions when coding our algorithms.

Let's say we are building a program to help us pick what to wear, but we hate the blue color. We could tell the computer to avoid that color using a statement like this:

1if color == 'blue':
2    # Do something
3else:
4    # Do something else or do nothing

👉 The switch statement is not available in Python.

What is a logical expression in Python?

Logical expressions let you conditionally skip a series of lines of your code. The best way to understand them is to think of them like questions you can ask the computer about your variables, for example:

1. if user_age > 21:
2. if day == "tuesday":
3. if car_model == "toyota" and number_of_tires == 6:

In order to ask a question or conditionally excuse a particular set of lines, you first need to have data (information) stored in useful variables. Above, we had the variables user_age, day, car_model and number_of_tires.

If we don't have the information pre-stored in variables, we are not able to ask any questions; it is all about strategy and planning!

For example, if we have the user's age stored in a variable age then, and only then, we are able to code for something like:

1# We use two equal signs (==) when we want to compare two variables for equality
2if age == 21:
3    print("You are old enough!")

What type of conditions/questions can we use/ask?

The previous example was a simple condition, but in real life, picking what to wear involves a combination of several conditions to make the final decision. For example: Let's look at this algorithm that tells you if you have the flu

If you want to represent this algorithm in Python, it will look something like this:

1feels_like_hit_by_train = True
2you_were_hit_by_train = False
3
4if feels_like_hit_by_train == True:
5    if you_were_hit_by_train == True:
6        print("You don't have the flu")
7    else:
8        print("You have the flu")
9else:
10    print("You don't have the flu")

Basically, this algorithm has two variables to consider: feels_like_hit_by_train and you_were_hit_by_train. Our job as developers is to sit down, try to prepare a strategy, and come up with an algorithm that solves a problem.

Logical Operators in Python

Most of the questions can be asked using the following comparisons: ==, >, <, !=, is None, is not None, in:

Operator	Example	Description
==	if a == b	if the value of variable a is equal to the value of b
<	if a < b	if the value of variable a is less than the value of b
>	if a > b	if the value of variable a is greater than the value of b
!=	if a != b	if the value of variable a is different from the value of b
is not None	if a is not None	This is self-explanatory, isn't it?
is None	if a is None	This is self-explanatory, isn't it?
in	if name in ['bob','maria','nancy']	If the value of name is contained inside the list of names

AND & OR Operators in Python

Another way to write the algorithm is to combine questions in the same condition using the AND and OR:

1feels_like_hit_by_train = True
2you_were_hit_by_train = False
3
4if feels_like_hit_by_train and you_were_hit_by_train:
5    print("You don't have the flu")
6elif feels_like_hit_by_train:
7    print("You have the flu")

As you can see we are using elif together for the first time, for faster coding. Another trick you can use for faster coding:

Original	Equivalent
instead of if(feels_like_hit_by_train == true)	you write if feels_like_hit_by_train
instead of if(you_were_hit_by_train == false)	you write if not you_were_hit_by_train

If...else in Python

You can also use the else expression to refer to the negation of the first condition:

1if color == "blue":
2    # Discard this clothing item
3else:
4    # Put it in your closet
5
6age = 12
7if age > 18:
8    print("You are an adult")
9else:
10    print("You are not an adult")

You can also nest several if...else conditions on top of one another, like this:

1if age < 16:
2    # You cannot do anything
3elif age < 18:
4    # At this point, we know it's older than 15 because if not, it would have entered into the first condition
5elif age < 21:
6    # If the algorithm enters here, we know it's older than 17
7else:
8    # If the algorithm enters here, we know it's older than 20

Here is another example that runs an algorithm to find out if a number is in the "hundreds".

1value = 14
2
3if value < 10:
4  print("single unit value")
5elif value < 100:
6  print("dozens")
7elif value < 1000:
8  print("hundreds")
9elif value < 10000:
10  print("thousands")
11else:
12  print("hundreds of thousands, or maybe more")

The switch statement in Python

Python does not have a switch statement.

Conclusion

It's all about what question to ask. The previous example was a simple condition, but real life is not that simple. There are lots of nested conditions and complicated flows that will challenge your skills to the limit. For example:

This will be the algorithm for picking what to wear on Valentine's Day:

1if going_out:
2    if can_I_get_burger:
3        if place_bottle_wine:
4            if cool_mix:
5                # Do something
6    else:
7        if blazers > 3:
8            # Do something
9        else:
10            # Do something
11    elif she_pants:
12        # Do something
13    else:
14        # Do something
15else:
16    if naked_she_door:
17        # Do something
18    elif blazers > 3:
19        # Do something
20    else:
21        # Do something