DataslopeControl Flow
if, elif, else, the conditional expression, and structural pattern matching
Every decision in software is a conditional. From "should I cache this result?" to "is the user authorized?" to "which HTTP status code do I return?", control flow is the engine that routes execution through different paths. Python's control flow is straightforward: if, elif, else, and (since Python 3.10) a match statement for pattern matching.
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Real-world impact
Conditionals are not just syntax — they are the logical heart of every program. A web server routes requests with if method == "GET". A data pipeline filters invalid rows with if is_valid(row). A game checks if player.health <= 0. You write conditionals hundreds of times a day. Mastering them means writing clear, correct, maintainable logic.
Just if
The simplest form: execute a block only if a condition is true.
If the condition is false, the block is skipped entirely.
if + elif
When you have multiple mutually exclusive conditions, use elif (short for "else if").
Python evaluates the conditions in order and stops at the first true one.
Even though x > 5 is also true, only "a" prints because the first branch matches.
Full if / elif / else
Add an else block to catch all remaining cases.
There can be any number of elif branches; else is optional. Each branch needs its own indented body (no braces).
Truthiness in conditions
The if condition is evaluated for truthiness, not strict equality with True. See the Booleans page for the full rules. In short: 0, "", [], {}, None, and False are falsy; everything else is truthy.
Use if items: to check for non-empty sequences. Use if x is None: for explicit None checks. Use if x: for general "does this have a meaningful value?" checks. All three are idiomatic.
Conditional expression (ternary)
When you need a value, not a statement, use the inline form:
<value_if_true> if <condition> else <value_if_false>This reads naturally: "pick X if condition, else pick Y." It combines nicely with f-strings and comprehensions.
The conditional expression requires both the if and else parts. You cannot write x if condition without an else. If you only need the if, write a full if statement instead.
Chained comparisons
Python lets you chain comparison operators, which usually reads more naturally than a > 0 and a < 10.
This is not syntactic sugar for separate comparisons; Python evaluates each term only once, which matters if the term has side effects.
match statement (Python 3.10+)
match is structural pattern matching, not just a switch. It matches the shape of data and can bind values from inside it.
Patterns can also match class instances, dict shapes, and sequences of varying lengths. See PEP 634 for the full reference.
Why no switch before 3.10?
Before Python 3.10, Python deliberately had no switch. The community preferred chained if/elif or, for value-to-value mappings, a dict lookup. The match statement was added not to replace those patterns, but to enable structural pattern matching — destructuring complex data shapes in one expression.
Dict-based dispatch (alternative to switch)
For simple value-to-value mappings, a dict is often clearer than if/elif or match.
This pattern is especially useful when the mapping is data-driven (e.g., loaded from a config file).
The walrus operator
Python 3.8 added the walrus operator :=, which assigns a value inside an expression. This is useful in conditionals:
if (n := len(items)) > 10:
print(f"Too many items: {n}")It saves a line by combining the assignment and the check. Use sparingly — only when it genuinely improves readability.
Challenges
Define a function fizzbuzz(n) that returns a list of length n where the i-th element (1-indexed) is:
"FizzBuzz"if i is divisible by both 3 and 5"Fizz"if i is divisible by 3 only"Buzz"if i is divisible by 5 only- The string form of i otherwise
A year is a leap year if it is divisible by 4, except century years (divisible by 100), which must also be divisible by 400. Define is_leap(year) returning True or False.
Define a function letter_grade(score) that returns the letter grade for a numeric score:
"A"for 90-100"B"for 80-89"C"for 70-79"D"for 60-69"F"for below 60
Assume score is an integer between 0 and 100.
What does this print?
x = 7
if x > 0:
print("a")
elif x > 5:
print("b")
else:
print("c")
a
b
a then b
c
Which match case matches a 2-tuple only when its two elements are equal (e.g. (4, 4))?
Hint: a guard is an if condition written after the pattern.
case (x, y):
case (x, y) if x == y:
case x and y:
case (x, x):
What is the value of result after this code?
x = 5
result = "big" if x > 10 else "small"
"big"
"small"
None
A SyntaxError
Which expression is equivalent to 0 < x < 10?
(0 < x) < 10
0 < x and x < 10
0 < (x < 10)
x in range(1, 10)
Next: looping.