Python Asserts

1. What is an Assertion

An assertion is a debugging aid that tests a condition and raises an error if the condition evaluates to False.

x = 10
assert x > 0

If the condition fails, Python raises an AssertionError.

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2. Basic Assertion Syntax

assert condition

Example:

age = 18
assert age >= 18

Program continues only if the condition is true.

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3. Assertion with Custom Message

score = 40
assert score >= 50, "Score must be at least 50 to pass"

Provides clear contextual error information.

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4. Assertion Failure Example

value = -5
assert value > 0, "Value must be positive"

Produces:

AssertionError: Value must be positive

Useful during validation and debugging phases.

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5. Using Assertions in Functions

def calculate_square(num):
    assert isinstance(num, int), "Input must be an integer"
    return num * num

print(calculate_square(5))

Ensures correct parameter usage.

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6. Assertions for Invariant Checks

def withdraw(balance, amount):
    assert amount <= balance, "Insufficient balance"
    return balance - amount

print(withdraw(500, 600))

Guarantees logical consistency during execution.

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7. Assertions vs Exception Handling

# Assertion
assert x != 0, "x must not be zero"

# Exception
if x == 0:
    raise ValueError("x must not be zero")

Assertion	Exception
Debugging aid	Production safety
Can be disabled	Always active
For internal logic	For user input validation

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8. Disabling Assertions in Production

Assertions can be disabled by running Python with optimization flag:

python -O script.py

This removes all assert statements from execution.

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9. Advanced Assertion with Complex Logic

def process_data(data):
    assert data is not None and len(data) > 0, "Data cannot be empty"
    return sum(data)

print(process_data([10, 20, 30]))

Combines multiple conditions in one assertion.

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10. Enterprise Use Case: Defensive Programming

def process_order(order):
    assert "id" in order, "Order must contain ID"
    assert order["amount"] > 0, "Order amount must be positive"
    return "Order Processed"

order = {"id": 101, "amount": 250}
print(process_order(order))

Ensures internal system contracts remain valid.

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Assertion Lifecycle

Stage	Role
Development	Detect logical errors early
Testing	Enforce internal rules
Production	Usually disabled or replaced by exceptions

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Best Practices

• Use assertions for internal logic validation

• Do not use for user-input validation

• Avoid side effects in assertions

• Keep conditions simple and readable

• Combine with logging for traceability

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Common Mistakes

• Using assert for runtime error handling

• Relying on assertions in production logic

• Overusing complex expressions in assertions

• Forgetting that assertions can be disabled

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Enterprise Importance

Assertions help:

• Detect programming errors early

• Maintain system integrity

• Validate algorithm invariants

• Reduce production issues

• Improve test reliability

Critical in:

• AI pipelines

• Financial computations

• Workflow validation engines

• Large-scale backend services

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