Documentation Index
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Introduction to Conditionals
Conditional statements allow your program to make decisions and execute different code blocks based on specific conditions. Python uses if, elif, and else keywords to create conditional logic.
Basic Conditional (IF)
The simplest form of conditional statement executes code only when a condition is True:
if 8 > 5:
print("8 es mayor que 5") # This will execute
Python uses indentation (typically 4 spaces) to define code blocks. All indented lines after the if statement belong to that conditional block.
IF-ELSE Statement
The if-else structure provides an alternative action when the condition is False:
helado = 'limon'
if helado == 'chocolate':
print('Sí es un helado de chocolate')
else:
print('No es un helado de chocolate') # This will execute
Structure
if condition:
# Code executed when condition is True
else:
# Code executed when condition is False
The else block is optional. Use it when you need to handle the case where the condition is not met.
IF-ELIF-ELSE Statement
For multiple conditions, use elif (else if) to check additional conditions:
puntaje = -4
if puntaje == 0:
print("El puntaje es neutro")
elif puntaje < 0:
print(f"El puntaje es negativo {puntaje}") # This will execute
elif puntaje > 0:
print(f"El puntaje es positivo {puntaje}")
else:
print("El puntaje no está dentro de los parámetros")
Structure
if condition1:
# Code for condition1
elif condition2:
# Code for condition2
elif condition3:
# Code for condition3
else:
# Code when none of the above conditions are True
You can have as many elif blocks as needed. Python evaluates them in order and executes the first one that’s True, then skips the rest.
Comparison in Conditionals
Conditionals work with comparison operators:
Equality Check
user_input = input("Enter a color: ")
if user_input == "blue":
print("You chose blue!")
else:
print("You chose a different color")
Numeric Comparisons
age = 18
if age >= 18:
print("You are an adult")
elif age >= 13:
print("You are a teenager")
else:
print("You are a child")
Multiple Conditions
score = 85
if score >= 90:
grade = "A"
elif score >= 80:
grade = "B" # This will execute
elif score >= 70:
grade = "C"
elif score >= 60:
grade = "D"
else:
grade = "F"
print(f"Your grade is: {grade}")
Practical Examples
Example 1: Temperature Checker
temperature = 25
if temperature > 30:
print("It's hot outside")
elif temperature > 20:
print("The weather is pleasant") # This will execute
elif temperature > 10:
print("It's cool outside")
else:
print("It's cold outside")
Example 2: Grade Evaluator
score = 75
if score >= 90:
print("Excellent! Grade: A")
elif score >= 80:
print("Very good! Grade: B")
elif score >= 70:
print("Good! Grade: C") # This will execute
elif score >= 60:
print("Passing. Grade: D")
else:
print("Failed. Grade: F")
Example 3: Sign Checker
number = -15
if number > 0:
print("The number is positive")
elif number < 0:
print("The number is negative") # This will execute
else:
print("The number is zero")
Example 4: Ice Cream Flavor Check
helado = 'limon'
if helado == 'chocolate':
print('Sí es un helado de chocolate')
else:
print('No es un helado de chocolate') # Output: No es un helado de chocolate
Nested Conditionals
You can place conditional statements inside other conditional statements:
age = 20
has_license = True
if age >= 18:
if has_license:
print("You can drive")
else:
print("You need a license to drive")
else:
print("You are too young to drive")
While nested conditionals work, consider using logical operators (covered in the next section) to make your code more readable.
Conditional Statement Flow
puntaje = -4
if puntaje == 0:
print("El puntaje es neutro")
elif puntaje < 0:
print(f"El puntaje es negativo {puntaje}") # ✓ Executes this
elif puntaje > 0: # ✗ Skips this even though we haven't checked it
print(f"El puntaje es positivo {puntaje}")
else: # ✗ Skips this too
print("El puntaje no está dentro de los parámetros")
Once a condition is met, Python executes that block and skips all remaining elif and else blocks. This is called “short-circuit evaluation.”
Best Practices
-
Order conditions from most specific to least specific
# Good
if score >= 90:
grade = "A"
elif score >= 80:
grade = "B"
# Bad - will never reach the second condition
if score >= 0:
grade = "Pass"
elif score >= 90:
grade = "A" # Never executes!
-
Use clear, descriptive conditions
# Good
if is_valid_user:
grant_access()
# Less clear
if x == True:
do_something()
-
Keep conditional blocks simple
- If a conditional block gets too long, consider extracting it into a function
- Use logical operators to combine simple conditions instead of nesting
Key Takeaways
if: Executes code when condition is Trueelif: Checks additional conditions if previous ones were Falseelse: Executes when all previous conditions are False- Python uses indentation to define code blocks
- Conditions are evaluated in order, and only the first
True condition executes
- You can have multiple
elif blocks but only one if and one else