ValueError: invalid literal for int()

Python ValueError "invalid literal for int()" catches developers off-guard when the string looks numeric but contains a decimal point, trailing whitespace with special characters, or locale-specific formatting. int('3.14') raises ValueError because int() cannot convert decimal strings — use int(float('3.14')) for this case. int(' 42 ') works (Python strips leading/trailing whitespace), but int('42') raises ValueError. Locale-aware number formats cause hard-to-trace failures. Many European locales format numbers with periods as thousands separators and commas as decimal separators: '1.234' means one thousand two hundred thirty-four in German formatting. User input or CSV files from European systems produce strings like '1.234' that fail int() for an unexpected reason — the period is a thousands separator, not a decimal point. Scientific notation ('1e3') is valid for float() but not for int() directly. Use int(float('1e3')) — float() parses scientific notation and int(float(...)) truncates correctly for integer values. The safest pattern for converting user input: strip whitespace, remove known thousands separators, validate with .lstrip('-').isdigit(), then convert with exception handling. For locale-aware parsing, the locale.atoi() function respects the system locale's numeric formatting conventions.