Conditionals

A conditional runs a block of code only when some condition is True — the same comparisons that produce a boolean (>, <, ==, and so on) are exactly what an if statement checks.

Keyword	Example	What it does
if	if length_ft > 10:	Runs its block only when the condition is True
elif	elif length_ft > 4:	Checked only if every condition above it was False — runs the first one that's True
else	else:	Catches everything not caught above — always comes last, with no condition of its own

If, Elif & Else

An if statement runs its indented block only when its condition is True — otherwise Python skips straight past it.

length_ft = 12

if length_ft > 10:
    print("that's a big snake")

Chaining Conditions With elif

length_ft = 12
if length_ft > 15:
    print("giant")
elif length_ft > 8:
    print("large")     # runs — 12 > 8, and nothing below it gets checked
elif length_ft > 4:
    print("medium")
else:
    print("small")

elif ("else if") is checked only if every condition above it was False. Python runs the first branch that matches and skips the rest, no matter how many elifs follow.

length_ft = 12
if length_ft > 15:
    print("giant")
elif length_ft > 8:
    print("large")
elif length_ft > 4:
    print("medium")
else:
    print("small")

length_ft = 3
if length_ft > 15:
    print("giant")
elif length_ft > 8:
    print("large")
elif length_ft > 4:
    print("medium")
else:
    print("small")

The else Clause

venomous = False
if venomous:
    print("handle with care")
else:
    print("safe to handle")    # runs — venomous is False

else is the catch-all — it runs when nothing above it matched. It always comes last and never has a condition of its own. A conditional chain can have elif without else, but else (if present) must always be the final branch.

venomous = False

if venomous:
    print("handle with care")
else:
    print("safe to handle")

Nested if Statements

length_ft = 12
venomous = False
if length_ft > 10:
    if venomous:
        print("big and dangerous")
    else:
        print("big but harmless")    # runs

An if can contain another if, checked only once the outer condition is already True — each level of nesting adds another layer of decision-making. If both conditions are simple, combining them with and (see below) is usually clearer than nesting.

length_ft = 12
venomous = False

if length_ft > 10:
    if venomous:
        print("big and dangerous")
    else:
        print("big but harmless")

Shorthand if

if venomous: print("careful")    # single-line if — the colon is still required

For a single statement, the body can go right on the same line as the condition. Save this for short, simple conditions — a normal multi-line if reads more clearly for anything more involved.

venomous = True
if venomous: print("careful")

Shorthand if-else (Ternary)

"careful" if venomous else "safe"    # "careful" — a compact if/else that evaluates to a value

This is a single expression, not a statement — it evaluates to one value or the other, so it's most useful for a quick assignment or a function argument, not as a stand-in for a full if block. value_if_true if condition else value_if_false reads almost like the English sentence it describes.

venomous = True
status = "careful" if venomous else "safe"
print(status)

The pass Statement

if venomous:
    pass    # placeholder — does nothing, but keeps the block syntactically valid

An empty block is a syntax error — pass is a no-op you can use as a placeholder while you're still deciding what a branch should do.

venomous = True

if venomous:
    pass

print("checked venomous status, no action taken yet")

Logical Operators

and, or, and not combine or invert conditions, so a single if can check more than one thing at once.

length_ft = 12
venomous = False

if length_ft > 10 and not venomous:
    print("big but safe to observe")

and

length_ft > 10 and venomous    # True and False → False

and needs both sides to be True for the whole condition to pass — if either side is False, the block is skipped.

length_ft = 12
venomous = False

if length_ft > 10 and venomous:
    print("big and dangerous")
else:
    print("not both conditions were true")

or

length_ft > 10 or venomous    # True or False → True

or only needs one side to be True — the block runs even if the other side is False.

length_ft = 12
venomous = False

if length_ft > 10 or venomous:
    print("worth a closer look")

not

not venomous    # not False → True

not flips a condition's result — useful for checking that something isn't the case, without rewriting the condition itself.

venomous = False

if not venomous:
    print("safe to handle")

Operator Precedence

length_ft > 10 and not venomous or length_ft > 20   # not, then and, then or

When mixing operators, Python evaluates not first, then and, then or. Parentheses aren't required, but they make the intended order obvious at a glance — (length_ft > 10 and not venomous) or length_ft > 20 means the same thing as the line above, with no guessing.

length_ft = 12
venomous = False

if (length_ft > 10 and not venomous) or length_ft > 20:
    print("worth a closer look")