str.zfill()

Added in v3.x · Updated March 13, 2026 · String Methods

stdlib string zfill

The .zfill() method pads a string on the left with zeros until it reaches the specified width. It’s particularly useful for formatting numbers, generating fixed-width identifiers, and ensuring consistent string lengths for display or processing.

Signature

str.zfill(width)

Parameters

Parameter	Type	Default	Description
`width`	`int`	—	The total width of the returned string. If the string is already longer than width, it’s returned unchanged.

Return Value

Returns a new string of the specified width, padded with leading zeros. If the original string’s length equals or exceeds the width, a new string (identical to the original) is returned.

Examples

Basic Usage

>>> "42".zfill(4)
'0042'

>>> "hello".zfill(10)
'00000hello'

The method adds zeros to the left until the string reaches the target width. Strings shorter than the width get padded; strings equal to or longer than the width are returned as-is.

Number Formatting

>>> "7".zfill(3)
'007'

>>> "123".zfill(5)
'00123'

>>> "12345".zfill(3)
'12345'  # No padding needed

This is the classic use case—formatting numbers with leading zeros, similar to what you might see in timestamps or invoice numbers.

Handling Negative Numbers

>>> "-42".zfill(5)
'-0042'

>>> "-3".zfill(4)
'-003'

.zfill() preserves the minus sign and pads after it. The total width includes the sign character.

Padding Hexadecimal and Other Bases

>>> hex(255)[2:].zfill(4)
'00ff'

>>> hex(4096)[2:].zfill(6)
'001000'

Combine with string slicing to pad hexadecimal representations.

Practical Use Cases

Generating sequential IDs:

for i in range(1, 11):
    print(f"ID-{str(i).zfill(3)}")
# Output:
# ID-001
# ID-002
# ...
# ID-010

Formatting timestamps:

from datetime import datetime

now = datetime.now()
print(f"{now.hour.zfill(2)}:{now.minute.zfill(2)}")
# Output: "09:05" (for 9:05)

File naming with sequential numbers:

for i in range(100):
    filename = f"data_{str(i).zfill(4)}.csv"
    # Creates: data_0000.csv, data_0001.csv, ... data_0099.csv

Behavior Details

How It Works

The method calculates how many zeros to prepend by subtracting the current string length from the target width:

>>> s = "42"
>>> width = 4
>>> zeros_needed = width - len(s)  # 4 - 2 = 2
>>> "0" * zeros_needed + s  # "00" + "42" = "0042"

Sign Handling

The sign (if present) is preserved at the front, and zeros are inserted after it:

>>> "+42".zfill(5)
'+0042'

>>> "-42".zfill(5)
'-0042'

>>> "+0".zfill(3)
'+00'

Non-Numeric Strings

.zfill() works on any string, not just numbers:

>>> "abc".zfill(6)
'000abc'

>>> "".zfill(3)
'000'

>>> "test".zfill(0)
'test'  # Width 0 returns original

Edge Cases

>>> "".zfill(0)
''

>>> "hello".zfill(-1)
'hello'  # Negative width returns original (Python 3.11+)

>>> "001".zfill(3)
'001'  # Already at width, no change

Common Mistakes

Assuming it works like rjust with ‘0’:

# zfill is equivalent to rjust with '0' for most cases:
>>> "42".zfill(4)
'0042'

>>> "42".rjust(4, '0')
'0042'

# But rjust can use any character:
>>> "42".rjust(4, 'x')
'xx42'

Forgetting about sign characters:

# The width includes the sign
>>> "-1".zfill(3)   # width=3 gives '-01' (sign + 2 digits)
'-01'

>>> "-1".zfill(4)   # width=4 gives '-001' (sign + 3 digits)  
'-001'

Using with floats:

# zfill doesn't work directly on floats
>>> "3.14".zfill(6)
'03.14'  # This works but may not be what you want

# For floats, convert to string first or use formatting:
>>> f"{3.14:05.2f}"
'03.14'

Performance

.zfill() creates a new string, so memory usage scales with the result size. It’s implemented in C and runs in O(n) time where n is the width. For most use cases with reasonable widths (under 1000 characters), performance is negligible.