Fix cast_to_bool to treat whitespace-only strings as False

[developreiloyo]

What does this change?

cast_to_bool was treating whitespace-only strings (e.g. " ", "\n") as True due to Python’s default bool(str) behavior.

This PR adds an explicit check to treat empty and whitespace-only strings as False.

What was done?

• Added a guard to return False for whitespace-only strings.
• Added a unit test covering this behavior.

Why this approach?

The change is minimal and does not alter existing behavior for numbers, YAML booleans, or standard truthy values.

[sisp]

Thanks for your contribution around bool casting improvements, @developreiloyo! 🙇 I've left a few inline comments.

[sisp]

Suggested change

	# Treat whitespace-only string as False
	if isinstance(value,str) and not value.strip():
	# Treat whitespace-only string as False
	if isinstance(value, str) and not value.strip():

[sisp]

Would it make sense to avoid the special case handling for whitespace-only strings and extend the string handling instead?

 with suppress(AttributeError):
-    lower = value.lower()
+    lower = value.lower().strip()
     if lower in {"y", "yes", "t", "true", "on"}:
         return True
-    elif lower in {"n", "no", "f", "false", "off", "~", "null", "none"}:
+    elif lower in {"n", "no", "f", "false", "off", "~", "null", "none", ""}:
         return False

[developreiloyo]

Thanks, @sisp! I included "" because without it, whitespace-only strings like " " fall through to bool(value), which returns True. Since .strip() normalizes them to "", treating it as a falsy keyword feels like the cleanest fix.

[sisp]

Isn't this the same as

lower = value.lower().strip()
if ...:
    ...
elif lower in {..., ""}:
    return False

but without a special case? " " would be captured by the elif branch.

[developreiloyo]

Thanks for the suggestion I realized the redundant check wasn’t needed, so I updated the implementation to use:

if isinstance(value, str) and not value.strip():
    return False

[sisp]

Ah, this is not what I meant. When we're at

copier/copier/_tools.py

Lines 130 to 136 in cd45f37

	# Assume it's a string
	with suppress(AttributeError):
	lower = value.lower()
	if lower in {"y", "yes", "t", "true", "on"}:
	return True
	elif lower in {"n", "no", "f", "false", "off", "~", "null", "none"}:
	return False

then the value is likely a string with methods like .lower() and also .strip(). So, I was thinking we only need to also strip leading and trailing whitespaces via .strip() and add the empty string to the set of values that shall cast to False. For this, I believe we don't need the special-case handling

if isinstance(value, str) and not value.strip():
    return False

but instead can extend the string handling logic just a little bit.

[sisp]

See #2462 (comment):

Suggested change

	# Handle whitespace-only strings first
	if isinstance(value, str) and not value.strip():
	return False