Python f-strings

f-strings evaluate expressions inline inside {} and format the result into the string.

name = "Dani"
f"Hello, {name}"

Conversion flags: !r, !s, !a

Append !r, !s, or !a before any :format_spec to control which conversion runs on the value before formatting, instead of the implicit str().

• !r calls repr() — quotes strings, shows None/booleans as their literal form, and reveals type-distinguishing detail ('1' vs 1).
• !s calls str() — the default, rarely needed explicitly.
• !a calls ascii() — like !r but escapes non-ASCII characters.

value = "1"
f"{value}"   # '1'
f"{value!r}" # "'1'"

Use !r in logging and debug output: it disambiguates a string from a number and shows None as None rather than an empty-looking value.

user_id = None
f"user_id={user_id!r}" # 'user_id=None'
f"user_id={user_id}"   # 'user_id=None' — same here, but not for e.g. "" vs None

Numeric format specs

Common :format_spec flags for numbers, after the ::

Flag	Effect	Example	Result
.Nf	fixed-point, N decimals	f"{3.14159:.2f}"	'3.14'
,	thousands separator	f"{1234567:,}"	'1,234,567'
_	thousands separator (underscore)	f"{1234567:_}"	'1_234_567'
%	multiply by 100, append %	f"{0.856:.1%}"	'85.6%'
+	force sign on positives	f"{42:+}"	'+42'
0Nd	zero-pad to width N	f"{7:03d}"	'007'
Nd/N	pad to width N (right-align)	f"{7:5d}"	' 7'
<N/>N/^N	left/right/center-align width N	f"{7:<5}"	'7 '
x/X	hex, lower/upper case	f"{255:x}"	'ff'
b	binary	f"{5:b}"	'101'
e	scientific notation	f"{12345:.2e}"	'1.23e+04'

Combine flags in order [align][sign][0][width][,/_][.precision][type], e.g. f"{-3.5:+08.2f}" → '-0003.50'.