Gotcha: Python f-strings with backslashes and expressions

Python f-strings have surprising limitations with backslashes and complex expressions that cause SyntaxError.

F-string limitations and workarounds:

# Backslashes NOT allowed inside f-string braces (before Python 3.12)
path = 'a\nb'
# BAD: SyntaxError
# f"Lines: {path.split('\n')}"  

# GOOD: Use a variable
newline = '\n'
f"Lines: {path.split(newline)}"

# Or assign the result first
lines = path.split('\n')
f"Lines: {lines}"

# Python 3.12+ allows backslashes in f-strings!
# f"Lines: {path.split('\n')}"  # Works in 3.12+

# Quotes inside f-strings
# BAD in Python < 3.12:
# f"{'hello'}"  # Same quote type inside

# GOOD: Use different quotes
f"{'hello'}"     # OK: different quotes
f'result: {d["key"]}'   # OK: different quotes
f"""result: {d['key']}"""  # OK: triple quotes

# Dictionary access in f-strings
d = {'name': 'Alice'}
f"Hello {d['name']}"    # Works with single inside double
# f"Hello {d["name"]}"  # SyntaxError before 3.12!

# Debugging with f-string = (Python 3.8+)
x = 42
f"{x = }"     # "x = 42"
f"{x = :.2f}" # "x = 42.00"
f"{len('hello') = }"  # "len('hello') = 5"

# Multi-line f-strings
message = (
    f"Name: {name}\n"
    f"Age: {age}\n"
    f"Email: {email}"
)

# Format spec in f-strings
f"{value:>10}"     # Right-align in 10 chars
f"{price:.2f}"     # 2 decimal places
f"{count:,}"       # Thousands separator: 1,234,567
f"{ratio:.1%}"     # Percentage: 85.3%
f"{n:#010x}"       # Hex with padding: 0x000000ff

F-string restrictions exist because the parser processes the string before evaluating expressions. Python 3.12 lifted most of these restrictions with PEP 701.

Python string formatting