Lesson 5 of 10

Module 5 · Python for Beginners

"Conditions: if, elif and else"

Make your code decide. Comparison and logical operators, if, elif and else, indentation, nested decisions, the one-line form, and which values count as true.

About 30 minutes

Open in ColabEvery example as a notebook you can run, edit and save to your Google Drive.

Comparisons give True or False

Every decision starts with a question that's either true or false. Comparison operators ask those questions:

Operator	Asks	Example	Result
`==`	equal to?	`5 == 5`	`True`
`!=`	not equal to?	`5 != 3`	`True`
`>`	greater than?	`5 > 3`	`True`
`<`	less than?	`5 < 3`	`False`
`>=`	greater than or equal?	`5 >= 5`	`True`
`<=`	less than or equal?	`4 <= 3`	`False`

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

True
False
False

Text comparisons are exact: capitals matter, so "Lagos" and "lagos" aren't equal.

if: do something only when it's true

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Low balance: top up soon
Done

The parts of an if:

if, then a condition, then a colon :.
The lines that belong to it are indented by four spaces. That's the block.
When the condition is true, the block runs. When it's false, Python skips it.
The first line back at the left edge (print("Done")) isn't part of the if: it always runs.

Indentation is part of the code

In most languages, indentation is just for looks. In Python, it decides which lines belong to the if:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Balance checked

Both indented lines were skipped because the condition was false. Colab indents for you when you press Enter after a colon. Use four spaces, and keep it consistent.

else: the alternative

else runs when the condition is false. Exactly one of the two blocks runs:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Fail: register for the resit

elif: several choices

elif ("else if") adds more conditions. Python checks them from the top and runs the first block whose condition is true, then skips the rest:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Score 67 is grade B

67 isn't >= 70, so Python moves on; it is >= 60, so grade is "B" and the remaining checks are skipped. That's why the order matters: put the highest band first. If score >= 50 came first, 67 would be graded C.

Combining conditions: and, or, not

Operator	True when	Example
`and`	both sides are true	`age >= 18 and has_id`
`or`	at least one side is true	`day == "Sat" or day == "Sun"`
`not`	the condition is false	`not is_paid`

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Eligible for the scholarship

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Weekend

Note that each side of or is a complete comparison. day == "Saturday" or "Sunday" looks right but is always true, a classic bug; the in operator below is a neater way to write it.

Ranges in one go

Python lets you chain comparisons the way you'd write them in maths:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Normal for Lagos

Checking membership with in

in checks whether a value is in a list (or a string):

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Weekend

Nested decisions

A block can contain another if. Indent once more for each level:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Fee: ₦20,000

More than two or three levels deep gets hard to read. Often and or elif does the same job more simply.

The one-line form

For a simple either-or value, there's a short form: value_if_true if condition else value_if_false:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Pass

Use it only when it's easy to read. For anything longer, write a full if/else.

Truthy and falsy values

if doesn't need a comparison. Any value can be tested, and Python treats some as false:

Treated as false	Everything else is true
`False`, `None`, `0`, `0.0`, `""` (empty text), `[]` (empty list)	`True`, any non-zero number, any non-empty text or list

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Please enter your name

That's a common way to check that someone actually typed something.

When it goes wrong

if score >= 50
    print("Pass")

Output

SyntaxError: expected ':'

Missing colon. Every if, elif and else line ends with :.

if score >= 50:
print("Pass")

Output

IndentationError: expected an indented block after 'if' statement on line 1

Missing indentation. The block must be indented.

if score = 70:
    print("Exactly 70")

Output

SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

= instead of ==. Python even suggests the fix.

Summary

You want to...	Write
run code only if true	`if cond:` + indented block
choose between two	`if cond: ... else: ...`
choose between several	`if ... elif ... elif ... else ...` (highest band first)
require both	`a and b`
accept either	`a or b` (full comparisons on both sides)
reverse a condition	`not a`
check a range	`low <= x <= high`
check a list	`x in [...]`
pick a value in one line	`a if cond else b`

Try it

Practice

Using the grading rules in the lesson (70+ A, 60+ B, 50+ C, 45+ D, otherwise F), what grade does a score of 46 get?

Practice

cgpa = 3.9 and attendance = 90. Is cgpa >= 4.0 and attendance >= 75 True or False?

Practice

What does "Pass" if 49 >= 50 else "Fail" give? Type it without quotes.

Task

8 min

Write a data plan advisor: store how many GB someone used last month in a variable, then use if, elif and else to recommend a plan (for example under 2 GB: Basic; 2 to 10 GB: Standard; over 10 GB: Premium), and print the recommendation with an f-string. Add one extra condition using and or or (for example, students get a discount). Paste your code.

Your work is checked for

if, elif and else, each ending with a colon
Blocks are indented
Uses comparisons like < or >=
Uses and or or
Prints with an f-string
Compares with == or >= / <=, not a single =

Earn your badge

Take the module check

Questions on this module. Pass and you earn the Python Decisions badge, free.

Create a free account to earn the badge

Lesson 5 of 10

Module 5 · Python for Beginners

"Conditions: if, elif and else"

Make your code decide. Comparison and logical operators, if, elif and else, indentation, nested decisions, the one-line form, and which values count as true.

About 30 minutes

Open in ColabEvery example as a notebook you can run, edit and save to your Google Drive.

Comparisons give True or False

Every decision starts with a question that's either true or false. Comparison operators ask those questions:

Operator	Asks	Example	Result
`==`	equal to?	`5 == 5`	`True`
`!=`	not equal to?	`5 != 3`	`True`
`>`	greater than?	`5 > 3`	`True`
`<`	less than?	`5 < 3`	`False`
`>=`	greater than or equal?	`5 >= 5`	`True`
`<=`	less than or equal?	`4 <= 3`	`False`

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

True
False
False

Text comparisons are exact: capitals matter, so "Lagos" and "lagos" aren't equal.

if: do something only when it's true

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Low balance: top up soon
Done

The parts of an if:

if, then a condition, then a colon :.
The lines that belong to it are indented by four spaces. That's the block.
When the condition is true, the block runs. When it's false, Python skips it.
The first line back at the left edge (print("Done")) isn't part of the if: it always runs.

Indentation is part of the code

In most languages, indentation is just for looks. In Python, it decides which lines belong to the if:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Balance checked

Both indented lines were skipped because the condition was false. Colab indents for you when you press Enter after a colon. Use four spaces, and keep it consistent.

else: the alternative

else runs when the condition is false. Exactly one of the two blocks runs:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Fail: register for the resit

elif: several choices

elif ("else if") adds more conditions. Python checks them from the top and runs the first block whose condition is true, then skips the rest:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Score 67 is grade B

Combining conditions: and, or, not

Operator	True when	Example
`and`	both sides are true	`age >= 18 and has_id`
`or`	at least one side is true	`day == "Sat" or day == "Sun"`
`not`	the condition is false	`not is_paid`

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Eligible for the scholarship

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Weekend

Note that each side of or is a complete comparison. day == "Saturday" or "Sunday" looks right but is always true, a classic bug; the in operator below is a neater way to write it.

Ranges in one go

Python lets you chain comparisons the way you'd write them in maths:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Normal for Lagos

Checking membership with in

in checks whether a value is in a list (or a string):

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Weekend

Nested decisions

A block can contain another if. Indent once more for each level:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Fee: ₦20,000

More than two or three levels deep gets hard to read. Often and or elif does the same job more simply.

The one-line form

For a simple either-or value, there's a short form: value_if_true if condition else value_if_false:

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Pass

Use it only when it's easy to read. For anything longer, write a full if/else.

Truthy and falsy values

if doesn't need a comparison. Any value can be tested, and Python treats some as false:

Treated as false	Everything else is true
`False`, `None`, `0`, `0.0`, `""` (empty text), `[]` (empty list)	`True`, any non-zero number, any non-empty text or list

Python · try it yourself

Edit the code and press Run (Ctrl + Enter). Examples on this page share their variables, like cells in a notebook.

Output

Please enter your name

That's a common way to check that someone actually typed something.

When it goes wrong

if score >= 50
    print("Pass")

Output

SyntaxError: expected ':'

Missing colon. Every if, elif and else line ends with :.

if score >= 50:
print("Pass")

Output

IndentationError: expected an indented block after 'if' statement on line 1

Missing indentation. The block must be indented.

if score = 70:
    print("Exactly 70")

Output

SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

= instead of ==. Python even suggests the fix.

Summary

You want to...	Write
run code only if true	`if cond:` + indented block
choose between two	`if cond: ... else: ...`
choose between several	`if ... elif ... elif ... else ...` (highest band first)
require both	`a and b`
accept either	`a or b` (full comparisons on both sides)
reverse a condition	`not a`
check a range	`low <= x <= high`
check a list	`x in [...]`
pick a value in one line	`a if cond else b`

Try it

Practice

Using the grading rules in the lesson (70+ A, 60+ B, 50+ C, 45+ D, otherwise F), what grade does a score of 46 get?

Practice

cgpa = 3.9 and attendance = 90. Is cgpa >= 4.0 and attendance >= 75 True or False?

Practice

What does "Pass" if 49 >= 50 else "Fail" give? Type it without quotes.

Task

8 min

Your work is checked for

if, elif and else, each ending with a colon
Blocks are indented
Uses comparisons like < or >=
Uses and or or
Prints with an f-string
Compares with == or >= / <=, not a single =

Earn your badge

Take the module check

Questions on this module. Pass and you earn the Python Decisions badge, free.

Create a free account to earn the badge