Common Type Mistakes

Unit ID: M02-U07 Estimated active time: 22-30 minutes

Diagnose before converting

When Python reports a type-related error, do not add random conversions until the code runs. First ask:

1. What value do I have?
2. What is its current type?
3. What operation am I asking for?
4. What type does that operation require?
5. Is conversion valid for the task?

Mistake 1: adding text and a number

planned_hours = "32"
extension_hours = 2
total_hours = planned_hours + extension_hours

Python raises TypeError. If "32" is valid numeric input, convert it:

total_hours = int(planned_hours) + extension_hours
print(total_hours)

Result: 34.

Do not convert extension_hours to text unless the real goal is to produce a label.

Mistake 2: joining output unsafely

score = 8
print("Score: " + score)

Use an f-string:

print(f"Score: {score}")

Mistake 3: confusing assignment and comparison

status = "available"
print(status == "available")

The first = assigns. The second == compares. Using = where a comparison is required causes a syntax error in ordinary expressions.

Mistake 4: using a method without calling it

raw_name = "  PYTHON  "
clean_name = raw_name.strip
print(clean_name)

This stores the method object instead of calling it. Add parentheses:

clean_name = raw_name.strip()
print(clean_name)

Mistake 5: indexing beyond the string

code = "M02"
print(code[3])

Valid positive indexes are 0, 1, and 2. Index 3 raises IndexError. Inspect len(code) and check the required position.

Mistake 6: changing case after comparison

entered_status = " AVAILABLE "
matches = entered_status == "available"
clean_status = entered_status.strip().lower()
print(matches)

The comparison happened before cleaning, so matches is False. Clean first, then compare:

clean_status = entered_status.strip().lower()
matches = clean_status == "available"

Execution order applies to transformations as well as variable creation.

Repair practice

Diagnose and repair:

raw_duration = " 3.5 "
duration = raw_duration.strip
next_duration = duration + 1
print("Next duration: " + next_duration)

A clear repair is:

raw_duration = " 3.5 "
clean_duration = raw_duration.strip()
duration = float(clean_duration)
next_duration = duration + 1
print(f"Next duration: {next_duration}")

Takeaway

Type errors usually reveal a mismatch between the value you have and the operation you requested. Inspect, explain, and then make the smallest valid conversion. You are now ready to clean and summarise a complete supplied record.