8.15 Common Mistakes

Invalid Conversions

Trying to int() a decimal-looking string directly fails, because int() expects a string that’s already a whole number — go through float() first if the string might have a decimal point.

>>> int("3.14")
Traceback (most recent call last):
ValueError: invalid literal for int() with base 10: '3.14'
>>> int(float("3.14"))    # correct two-step approach
3

Data Loss

Converting float to int silently discards the fractional part (see 8.4 Numeric Type Conversion) — easy to miss if not paying attention, since no error or warning is raised.

>>> price = 19.99
>>> int(price)      # silently loses the cents -- no error at all
19

Incorrect Boolean Conversion

What Goes Wrong?

Assuming bool("False") or bool("0") returns False, because the text looks like a negative value. In reality, any non-empty string is truthy, regardless of its content.

>>> bool("False")     # non-empty string -- ALWAYS True
True
>>> bool("0")          # also True -- a very common gotcha
True

# Correct way to parse a boolean-looking string:
>>> value = "False"
>>> value.lower() == "true"
False

Quick Interview Answer

“Three conversion bugs come up constantly: calling int() directly on a decimal-looking string, which raises ValueError because int() only accepts strings that are already whole numbers — the fix is int(float(x)); silently losing precision converting float to int, since truncation raises no warning at all; and assuming bool() parses boolean-looking text semantically — bool(\"False\") and bool(\"0\") are both True, because any non-empty string is truthy regardless of content. Parsing an actual boolean-looking string correctly means comparing its lowercased text against \"true\" explicitly, not calling bool() on it.”

Common Mistakes

• Calling bool(some_string) to parse a config value that’s textually "true"/"false" — it always returns True for any non-empty string; compare the lowercased text instead.
• Not noticing int(19.99) silently drops the cents — worth an explicit round() first if the intent is rounding rather than truncation.
• Skipping the float() intermediate step when parsing a decimal string with int(), hitting an avoidable ValueError.