str.zfill()

Zero-pad numbers-as-strings: IDs, timestamps, sortable filenames.

String methodPython 2.2+Live demo
Common call
"42".zfill(5)
Returns
new str, length ≥ width
Replaces
sign-aware: "-42".zfill(5) → "-0042"
Watch out
already-long strings come back unchanged — never truncated
str.zfill(widthwidth — Target minimum length. Shorter strings are padded; longer ones returned unchanged.type: int · required)
→ str

Demo

Live evaluation
Try:
Inputs
stringstrthe source
widthinttarget width
Output
'42'.zfill(5)
'00042'

Zeros are inserted after any leading sign, before the rest. A string already at or past the width is returned untouched — zfill never truncates. It pads any string, not just digits.

Parameters

NameTypeRequiredDescription
widthintyesTarget minimum length. Shorter strings are padded; longer ones returned unchanged.

Return value

str — A copy left-padded with ASCII zeros to at least width characters. A leading sign stays in front of the padding.

Common patterns

Sortable numeric filenames
Zero-padding makes lexicographic order match numeric order.
name = f"img_{str(i).zfill(4)}.png"
# img_0001.png … img_0042.png
Fixed-width IDs and codes
Invoice numbers, ZIP codes read from ints, etc.
invoice = str(n).zfill(8)
The f-string alternative
For numbers you are formatting anyway, :0Nd does it inline.
f"{i:04d}"   # '0042' — same result, no str() call

Examples

1. Basic zero-padding
"42".zfill(5)
Returns
'00042'
2. Sign stays in front
"-42".zfill(5)
Returns
'-0042'
3. Longer than width
"123456".zfill(3)
Returns
'123456'
4. Works on any string
"ab".zfill(4)
Returns
'00ab'

Pitfalls

1. zfill pads — it never truncates
Fixed-width output needs an explicit slice for the overflow case.
Still 6 chars
"123456".zfill(3)
'123456'
If truncation is wanted
s.zfill(3)[-3:]
'456' — explicit choice
2. Only zfill is sign-aware — rjust is not
Padding with rjust(5, "0") puts zeros BEFORE the minus sign.
Broken number
"-42".rjust(5, "0")
'00-42'
Fix
"-42".zfill(5)
'-0042'

When to use

Use it
  • Zero-padding strings you already have
  • Sortable numeric filenames and IDs
Reach for something else
  • Formatting a number directly → f"{n:04d}"
  • Padding with other characters → str.rjust(width, char)
  • Centering → str.center

Notes

Complexity
O(width)
Return
new str — source untouched
CPython impl
Objects/unicodeobject.c :: unicode_zfill
Memory
One new string of max(len, width)
Thread-safe
Yes — str is immutable

FAQ

If you have a number, format it directly with f"{n:04d}". zfill earns its keep when you already have a string (IDs read from files, user input).

History

2.2
Method added to str.