Making decisions with if, elif and else · Python tutorial

So far every line has run, top to bottom. Real programs choose: a discount only for members, an error only when input is wrong. That is what if is for.

if#

temperature = 31

if temperature > 30:
    print("It's hot")
    print("Drink water")
print("Done")

The condition ends with a colon, and the lines that belong to it are indented. Both lines under if run only when the condition is true; Done runs either way, because it is back at the left edge.

else#

if temperature > 30:
    print("It's hot")
else:
    print("It's fine")

Exactly one of the two branches runs.

elif#

For more than two choices, chain with elif (short for "else if"):

marks = 72

if marks >= 90:
    grade = "A"
elif marks >= 75:
    grade = "B"
elif marks >= 60:
    grade = "C"
else:
    grade = "F"

print(grade)    # C

Python checks each condition in order and runs the first one that is true, then skips the rest. That is why the order matters: put >= 60 first and every passing mark becomes a C.

Combining conditions#

age = 25
has_id = True

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

if day == "Saturday" or day == "Sunday":
    print("Weekend")

if not has_id:
    print("Bring ID")

Membership with in reads well for several values:

if day in ("Saturday", "Sunday"):
    print("Weekend")

What counts as true#

Any value can be used as a condition. These count as false: False, None, 0, 0.0, "" (empty string), [] (empty list), {} (empty dict). Everything else is true.

name = input("Name: ")
if name:
    print(f"Hi {name}")
else:
    print("You didn't type anything")

if name: reads "if there is a name". It is the usual Python way to check for empty text or an empty list.

Nested decisions#

An if can sit inside another, indented one more level:

if logged_in:
    if is_admin:
        print("Admin panel")
    else:
        print("Dashboard")
else:
    print("Please sign in")

Deep nesting gets hard to read. Often you can flatten it by handling the special case first:

if not logged_in:
    print("Please sign in")
elif is_admin:
    print("Admin panel")
else:
    print("Dashboard")

The conditional expression#

For choosing between two values, there is a one-line form:

label = "even" if n % 2 == 0 else "odd"

Read it as "even if n is divisible by two, else odd". Use it for short choices of a value; use a full if when there is more to do.

Common mistakes#

Forgetting the colon at the end of if, elif or else.
Indenting one line of a block differently from the others.
if x = 5: instead of if x == 5:. Python refuses the first, which is helpful.
Ordering elif checks so an earlier, wider one swallows a later, narrower one.

Key points#

if, elif and else choose which block runs; only the first true branch does.
Blocks are marked by a colon and indentation.
Empty values and zero count as false.
a if condition else b picks between two values in one line.

Next: doing things more than once, with loops.

if#