Common Pitfalls0%
List Introduction

Common Pitfalls

Beginner8 min read•Updated: 2026-09-03
Study Materials

Common Pitfalls

Understanding common type-casting pitfalls prevents subtle bugs and runtime crashes.

Key Concepts & Detailed Explanation

Top 3 Casting Pitfalls in Python:

  1. 1
    ValueError on int(float_string): int("3.14") crashes! You must do int(float("3.14")).
  2. 2
    The bool("False") Trap: Any non-empty string in Python evaluates to True! Therefore bool("False") is True.
  3. 3
    Float Arithmetic Precision: Floating-point numbers cannot represent some base-10 fractions exactly: 0.1 + 0.2 != 0.3 (it equals 0.30000000000000004).

Code Examples & Output

Python
# Trap 1: Non-integer string to int
try:
bad_int = int("45.8")
except ValueError as e:
print("Caught expected ValueError:", e)
 
# Correct way
good_int = int(float("45.8"))
print("Proper conversion:", good_int)
 
# Trap 2: bool("False") is True!
print("bool('False'):", bool("False")) # True!
print("bool(''):", bool("")) # False (empty is falsy)

Expected Output:

Output
Caught expected ValueError: invalid literal for int() with base 10: '45.8'
Proper conversion: 45
bool('False'): True
bool(''): False

Best Practices & Common Pitfalls

To safely parse a boolean from text, check if text.strip().lower() in ('true', '1', 'yes').

Practice Quiz

1. What is the output of bool('False') in Python?

  • A) False
  • B) True
  • C) ValueError
  • D) None
Answer: B Explanation: Any non-empty string is truthy, so bool('False') evaluates to True.

2. Why does int('12.34') raise a ValueError?

  • A) Python doesn't support decimals
  • B) The int() function expects pure digit characters when parsing strings
  • C) String is too long
  • D) Only floats exist in Python 3
Answer: B Explanation: int() expects an integer literal when converting from string.

Practice Challenge

Write a safe parsing function 'safe_to_int(val, default=0)' that returns the default if conversion fails.

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