Conditionals#

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What a conditional actually is#

Every program you've used makes decisions. A streaming app decides whether to show a "resume watching" button based on whether you've started a show. A banking app decides whether to let a withdrawal go through based on your balance. A conditional is how you write "if this is true, do this, otherwise do that" in code.

Think about ordering coffee at a counter. If you ask for it iced, they grab a cold cup. If you ask for it hot, they grab a different cup. There's no single fixed action; the action taken depends on a condition (what you asked for) being checked first. That's exactly what a conditional does in code: check something, then branch into one path or another based on the answer.

The basic shape: if, elif, else#

temperature = 15

if temperature > 25:
    print("It's hot.")
elif temperature > 10:
    print("It's mild.")
else:
    print("It's cold.")

Read this top to bottom, the way Python actually executes it: check the first condition (temperature > 25). If that's true, run the indented code under it and skip everything else in this chain entirely, even if a later condition would also have been true. If it's false, move to the elif ("else if") and check that condition instead. If none of the if/elif conditions were true, fall through to else, which has no condition of its own; it just means "none of the above."

You can have as many elif blocks as you need, and both elif and else are optional. A plain if with nothing else is completely valid if you only care about one case.

Indentation is not a style choice, it's the syntax#

In many languages, curly braces {} mark which lines belong inside an if block. Python uses indentation (whitespace at the start of a line) for that instead. The lines indented under if temperature > 25: are the lines that run when that condition is true. This is not a formatting preference, it's how Python parses your code. Get the indentation wrong, and Python either throws an IndentationError or, worse, runs a different block than you intended without any error at all.

Two practical rules to avoid the most common pain here:

• Pick one indentation style, four spaces per level is the near-universal Python convention, and let your editor's Python settings enforce it consistently. VS Code, once it detects a .py file, will typically indent for you correctly when you press Enter after a :.
• Never mix tabs and spaces in the same file. Python will sometimes tolerate this and sometimes throw a confusing TabError, depending on exactly how they're mixed. Avoid the whole problem by never mixing them.

Also notice the colon : at the end of the if, elif, and else lines. Forgetting it is one of the single most common beginner typos, and it produces a SyntaxError pointing roughly at that line. If you see a SyntaxError right after writing a conditional, check for a missing colon before anything else.

Comparison operators#

These are the building blocks of a condition, and they produce a bool (True or False):

The one to burn into memory: == checks equality, = assigns a value. They look almost identical and do completely different things. if score = 10: is a syntax error in Python (it will refuse to run), which is actually a kindness, some other languages let you write that and silently do the wrong thing. Even so, read every condition you write once, slowly, to confirm you meant == and not =.

Combining conditions: and, or, not#

Sometimes one comparison isn't enough. You can combine multiple conditions:

age = 20
has_ticket = True

if age >= 18 and has_ticket:
    print("Allowed in.")

and requires both sides to be true. or requires at least one side to be true. not flips a boolean: not True is False.

A common beginner mix-up is reaching for and when the situation actually calls for or, or vice versa. If you want "let this person in if they're either a member or have a guest pass," that's is_member or has_guest_pass. Using and there would wrongly exclude non-members who have a valid guest pass. When a condition isn't behaving the way you expect, one of the first things to check is whether you actually want and where you wrote or, or the reverse. Say the rule out loud in plain English first, then translate it, rather than guessing.

Truthy and falsy values#

You'll sometimes see conditions that don't use a comparison operator at all:

name = ""

if name:
    print(f"Hello, {name}")
else:
    print("No name given.")

Python treats certain values as automatically "falsy" when used directly in a condition, even though they aren't literally False: an empty string "", the number 0, None, and empty collections like [] or {}. Everything else is "truthy." So if name: really means "if name is not empty." This is convenient once you're used to it, but it can be genuinely confusing the first time you see it, since nothing about "" looks like it should behave like False. If a conditional is behaving strangely and you're not using an explicit comparison, check whether you're accidentally relying on truthiness when you meant to check something more specific, like if name != "":.

What to watch for#

• Missing colon at the end of if/elif/else lines.
• Inconsistent or mixed indentation (tabs and spaces together).
• Writing = where you meant == inside a condition (Python will stop you here with a SyntaxError, which is a hint something's wrong, not an obstacle).
• Reaching for and when the logic actually needs or, or the reverse. Say the rule in plain English before writing the code.
• Relying on truthiness (if name:) without realizing that 0, "", and empty collections all count as falsy, which can produce surprising results if one of those is a legitimate value you meant to treat as "present."

TL;DR#

A conditional lets your code check something and run different code depending on whether it's true or false. This page covers the if/elif/else structure, how indentation and comparisons work, and how to combine or evaluate conditions.

• if, elif ("else if"), and else let you check a condition and branch into exactly one path; only the first true condition's block runs.
• Indentation, not curly braces, is what tells Python which lines belong inside each branch. Get it wrong, and Python either throws an error or runs the wrong block.
• Comparison operators like ==, >, and < each produce True or False. == checks equality, = assigns a value, don't mix them up.
• and needs every condition to be true, or needs just one, and not flips a True/False value.
• Python also treats some values, like an empty string "", 0, or None, as automatically false in a condition, even without an explicit comparison.

books_checked_out = 2
is_member = True
username = ""

if books_checked_out > 5:                 # if/elif/else: check a condition, run one path
    print("Checkout limit reached.")
elif books_checked_out > 0:               # indentation (not braces) marks what's "inside" this branch
    print("Some books are checked out.")
else:
    print("No books checked out.")

if books_checked_out > 0 and is_member:   # ">" and "and" both produce True/False
    print("Enjoy your books.")

if not username:                          # "" is falsy, so "not username" is True here
    print("Please enter a username.")