How to Pad Number with Leading Zeros in Python

In Python, you can pad a number with leading zeros using str.zfill(width), format() with format specifiers like "{:0widthd}", or f-strings such as f"{num:0{width}d}". These methods convert the number to a string with zeros added at the start to reach the desired length.

Syntax

Here are common ways to pad numbers with leading zeros in Python:

• str.zfill(width): Pads the string with zeros on the left to reach width.
• format() or f-strings: Use format specifiers like "{:0widthd}" where width is the total length.

Replace width with the total number of digits you want.

python

num = 42

# Using zfill
str_num = str(num).zfill(5)  # '00042'

# Using format()
str_num2 = "{:05d}".format(num)  # '00042'

# Using f-string
str_num3 = f"{num:05d}"  # '00042'

Example

This example shows how to pad the number 7 with leading zeros to make it 4 digits long using three methods.

python

num = 7
width = 4

# Using zfill
padded1 = str(num).zfill(width)

# Using format()
padded2 = "{:0{}d}".format(num, width)

# Using f-string
padded3 = f"{num:0{width}d}"

print(padded1)
print(padded2)
print(padded3)

Output

0007
0007
0007

Common Pitfalls

Common mistakes include:

• Trying to pad numbers directly without converting to string (only string methods like zfill() work).
• Using incorrect format specifiers that don't pad zeros.
• Confusing the width parameter with the number of zeros to add (width is total length).

Always ensure you convert numbers to strings if using string methods, or use proper format specifiers.

python

num = 5

# Wrong: zfill on int (raises error)
# padded_wrong = num.zfill(3)  # AttributeError

# Right:
padded_right = str(num).zfill(3)  # '005'

# Wrong: format without zero padding
padded_wrong2 = "{:3d}".format(num)  # '  5' (spaces, not zeros)

# Right:
padded_right2 = "{:03d}".format(num)  # '005'

Quick Reference

Method	Syntax	Description
zfill	str(num).zfill(width)	Pads string with zeros to total length width
format()	"{:0widthd}".format(num)	Formats number with leading zeros to width
f-string	f"{num:0{width}d}"	Same as format(), modern and readable

Key Takeaways

Use str.zfill(width) to pad numbers converted to strings with leading zeros.

Format specifiers like {:0widthd} in format() or f-strings add leading zeros easily.

Width means total length including the number, not just zeros to add.

Always convert numbers to strings before using string methods like zfill.

f-strings provide a clean and modern way to pad numbers with zeros.