🖨️ Print Debugging

Print debugging is the simplest debugging technique: add print() statements to see what your code is doing. Despite being "primitive," it's often the fastest way to find bugs.

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✅ Basic Print Debugging

def calculate_total(items):
    print(f"Input items: {items}")  # What did we receive?

    total = 0
    for item in items:
        print(f"  Processing: {item}")  # What are we iterating?
        total += item['price'] * item['quantity']
        print(f"  Running total: {total}")  # State after each step

    print(f"Final total: {total}")  # What are we returning?
    return total

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✅ What to Print

1. Function inputs - Are arguments what you expected?
2. Loop iterations - Is the loop running the right number of times?
3. Conditional branches - Which branch is being taken?
4. Variable state - What values do variables hold?
5. Function outputs - What's being returned?

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✅ Formatting Debug Output

# Include variable names (f-strings with =)
x = 42
print(f"{x=}")  # Output: x=42

# Include types
data = [1, 2, 3]
print(f"data={data!r}, type={type(data).__name__}")

# Add context/labels
print(f"[DEBUG] After loop: count={count}, total={total}")

# Use separators for visibility
print("=" * 40)
print(f"CHECKPOINT: {checkpoint_name}")
print("=" * 40)

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✅ Debug Print Patterns

# Pattern 1: Entry/Exit tracing
def process(data):
    print(f">>> Entering process({data!r})")
    result = do_something(data)
    print(f"<<< Exiting process() -> {result!r}")
    return result

# Pattern 2: Conditional debug
DEBUG = True
if DEBUG:
    print(f"[DEBUG] value={value}")

# Pattern 3: Quick type check
print(f"{type(mystery_var)=}")

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✅ Using Logging Instead of Print

For production code, use logging instead of print:

import logging

logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger(__name__)

def process(data):
    logger.debug(f"Processing {data}")
    # ...
    logger.info(f"Processed {len(data)} items")

Advantages: - Can be turned on/off without code changes - Has levels (DEBUG, INFO, WARNING, ERROR) - Can output to files, not just console

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✅ When to Use Print Debugging

Good for: - Quick investigations - Understanding code flow - Simple bugs - Learning new code

Consider alternatives when: - Bug is complex or intermittent - Need to inspect many variables - Code is hard to re-run

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🔍 Key Takeaways

• Print debugging is simple but powerful.
• Use f-strings with = for quick variable inspection.
• Add context (labels, separators) to make output readable.
• Consider logging for production code.
• Remove debug prints before committing!

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