Ben Warai Otoko

Python Control Flow (if/elif/else)

7 min read

Python Control Flow (if/elif/else)

Control flow is how your program stops being a linear script and starts making decisions. if/elif/else sounds trivial until you realize it’s the backbone of every validation check, every routing rule, every data quality gate in your pipeline. Get this right and your code reads like clear logic. Get it wrong and you end up with deeply nested conditions nobody — including you — wants to debug.

Basic Syntax

If Statement

if condition:
    # This block executes only if condition is True
    print("Condition was true")

If-Else Statement

if condition:
    # Executes if condition is True
    print("Condition was true")
else:
    # Executes if condition is False
    print("Condition was false")

If-Elif-Else Chain

if condition1:
    # Executes if condition1 is True
    print("First condition was true")
elif condition2:
    # Executes if condition1 is False AND condition2 is True
    print("Second condition was true")
elif condition3:
    # Executes if condition1 and condition2 are False AND condition3 is True
    print("Third condition was true")
else:
    # Executes if all previous conditions are False
    print("None of the conditions were true")

Key point: Only ONE block executes. Python checks conditions top-to-bottom and stops at the first True condition.

Quick Reference Examples

Simple If-Else

age = 18
 
if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")
# Output: You are an adult

If-Elif-Else Chain

score = 75
 
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"
 
print(f"Your grade is {grade}")  # Your grade is C

Multiple Conditions with Logical Operators

age = 25
salary = 60000
 
# AND: both conditions must be True
if age >= 21 and salary >= 50000:
    print("Eligible for premium membership")
# Output: Eligible for premium membership
 
# OR: at least one condition must be True
if age < 18 or salary < 30000:
    print("Not eligible for loan")
else:
    print("Eligible for loan")
# Output: Eligible for loan
 
# NOT: reverses the condition
if not (age < 18):
    print("You are 18 or older")
# Output: You are 18 or older

Membership Testing

fruits = ["apple", "banana", "cherry"]
user_choice = "banana"
 
if user_choice in fruits:
    print(f"{user_choice} is available")
else:
    print(f"{user_choice} is not available")
# Output: banana is available
 
# Check if value is NOT in list
if "grape" not in fruits:
    print("We don't have grapes")
# Output: We don't have grapes

Ternary Operator (Conditional Expression)

# One-line if-else for simple assignments
age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # adult
 
# Works with any expression
score = 85
result = "Pass" if score >= 60 else "Fail"
print(result)  # Pass
 
# Chained ternary (use with caution—can become unreadable)
temperature = 25
weather = "hot" if temperature > 30 else "warm" if temperature > 20 else "cold"
print(weather)  # warm

Boolean Operators (Logical Operators)

AND Operator

and returns True only if both conditions are true.

age = 25
license = True
 
if age >= 18 and license:
    print("You can drive")  # This executes
else:
    print("You cannot drive")
 
# AND with multiple conditions
if age >= 21 and license and license != False:
    print("Eligible for everything")

Short-circuit evaluation: Python stops evaluating as soon as it finds a False. If the first condition is false, it doesn’t check the rest.

def check_age():
    print("Checking age...")
    return False
 
def check_license():
    print("Checking license...")
    return True
 
if check_age() and check_license():
    print("Can drive")
 
# Output:
# Checking age...
# (Note: check_license is NOT called because check_age was False)

OR Operator

or returns True if at least one condition is true.

day = "Saturday"
 
if day == "Saturday" or day == "Sunday":
    print("It's the weekend!")  # This executes
else:
    print("It's a weekday")
 
# OR with multiple conditions
if temperature < 0 or temperature > 40:
    print("Extreme weather warning")

NOT Operator

not reverses a boolean value.

is_raining = False
 
if not is_raining:
    print("Let's go outside")  # This executes
else:
    print("Stay inside")
 
# NOT with conditions
if not (age < 18):
    print("You are 18 or older")
 
# Avoid double negatives (not recommended)
# if not not is_raining:  # ❌ confusing
if is_raining:  # ✅ clearer
    print("It's raining")

Comparison Operators

These return True or False and are commonly used in conditions.

OperatorMeaningExample
==Equal tox == 5
!=Not equal tox != 5
<Less thanx < 5
>Greater thanx > 5
<=Less than or equalx <= 5
>=Greater than or equalx >= 5
inMember of sequence"a" in "apple"
not inNot in sequence3 not in [1, 2, 4]
isSame object (identity)x is None
is notNot same objectx is not None

Common Patterns in Data Engineering

Validating Input

def process_record(data: dict) -> bool:
    """Validate record before processing."""
    if "id" not in data:
        print("Error: Missing ID")
        return False
    
    if not isinstance(data["id"], int):
        print("Error: ID must be integer")
        return False
    
    if data["id"] <= 0:
        print("Error: ID must be positive")
        return False
    
    return True

Handling Missing Values

temperature = None
 
if temperature is None:
    temperature = 20.0  # Default value
    print(f"Using default temperature: {temperature}")
 
# Or more concise:
temperature = temperature or 20.0

Routing Based on Data Type

value = "42"
 
if isinstance(value, int):
    result = value * 2
elif isinstance(value, str):
    result = int(value) * 2
elif isinstance(value, float):
    result = int(value) * 2
else:
    result = None
 
print(result)  # 84

Filtering Records

records = [
    {"name": "Alice", "age": 30, "active": True},
    {"name": "Bob", "age": 25, "active": False},
    {"name": "Charlie", "age": 35, "active": True},
]
 
# Filter active adults
valid_records = [
    r for r in records 
    if r["active"] and r["age"] >= 18
]

Tips & Gotchas

  • Use == for value comparison, not is. is checks if two variables point to the same object in memory. x == 5 asks “are the values equal?” while x is 5 asks “are they the exact same object?”
a = [1, 2, 3]
b = [1, 2, 3]
 
a == b  # True (same contents)
a is b  # False (different objects in memory)
  • None should be checked with is None, not == None. By convention, use if x is None: rather than if x == None:.
x = None
 
if x is None:  # ✅ Pythonic
    print("x is None")
 
if x == None:  # ⚠️ Works, but not conventional
    print("x is None")
  • Truthiness vs explicit comparison. Don’t rely on truthiness when you mean to compare values.
# ❌ What does "if items:" mean? Is it checking for existence?
if items:
    process(items)
 
# ✅ Explicit is better than implicit
if items is not None and len(items) > 0:
    process(items)
 
# ✅ Or use explicit length check
if len(items) > 0:
    process(items)
  • Avoid deeply nested if-else. If you have more than 2–3 levels, consider using a dictionary lookup or early returns.
# ❌ Deeply nested (hard to read)
if condition1:
    if condition2:
        if condition3:
            do_something()
 
# ✅ Use early returns (clearer)
if not condition1:
    return
if not condition2:
    return
if not condition3:
    return
do_something()
  • Use elif instead of multiple if statements when conditions are mutually exclusive. It’s clearer and slightly faster.
# ❌ Multiple if statements (all are evaluated)
if x > 0:
    print("positive")
if x == 0:
    print("zero")
if x < 0:
    print("negative")
 
# ✅ If-elif-else (stops at first match)
if x > 0:
    print("positive")
elif x == 0:
    print("zero")
else:
    print("negative")
  • Ternary operator is readable up to one level. Beyond that, use if-elif-else.
# ✅ Readable
status = "pass" if score >= 60 else "fail"
 
# ⚠️ Getting hard to read
grade = "A" if score >= 90 else "B" if score >= 80 else "C" if score >= 70 else "F"
 
# ✅ Use if-elif-else instead
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"
  • Empty lists/dicts are falsy; non-empty are truthy. But be explicit.
items = []
 
# ❌ Implicit truthiness check
if not items:
    print("No items")
 
# ✅ Explicit (clearer intent)
if len(items) == 0:
    print("No items")

Key Takeaway:
Control flow is decision-making. Use if-elif-else for multiple paths, boolean operators (and, or, not) for complex conditions, and ternary operators for simple assignments. Keep conditions readable—your future self will thank you.

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