Gotcha: Python f-string cannot contain backslash in expression

Using backslash characters inside f-string curly braces raises a SyntaxError.

Backslashes are not allowed in f-string expressions:

# BAD - SyntaxError:
f"{'\n'.join(items)}" # Can't use \n in {}
f"{'\t'.join(items)}" # Can't use \t in {}
f"{path.replace('\\', '/')}" # Can't use \\ in {}

# GOOD - assign to variable first:
newline = '\n'
f"{newline.join(items)}"

# GOOD - use chr():
f"{chr(10).join(items)}" # chr(10) = newline

# GOOD - use a function:
def join_lines(items):
return '\n'.join(items)
f"{join_lines(items)}"

# GOOD - use str.join outside f-string:
result = '\n'.join(f"{item}: {value}" for item, value in pairs)

# Python 3.12+ FIXES THIS:
# f"{'\n'.join(items)}" # Works in Python 3.12+!

# Other f-string limitations (fixed in 3.12):
# - No reuse of same quote type
# f"{'hello'}" # OK
# f"{'he"llo'}" # SyntaxError before 3.12
# - No nested f-strings with same quotes
# f"{f'{x}'}" # SyntaxError before 3.12

Before Python 3.12, the f-string parser couldn't handle backslashes in expressions. Python 3.12 rewrote the parser to lift this restriction.