Python String zfill() Method

The zfill() method in Python is used to pad a numeric string with leading zeros, so that the resulting string reaches a specified length. It's especially useful when formatting numbers like invoice IDs, codes, or timestamps.

Syntax

string.zfill(width)

Parameter:

• width – The total length of the resulting string (including leading zeros).

Returns:

• A copy of the string left-padded with zeros to make it at least width characters long.

Example 1: Simple Zero Padding

text = "42"
print(text.zfill(5))

00042

Example 2: zfill() with Negative Numbers

text = "-42"
print(text.zfill(5))

-0042

Note: The negative sign is preserved in its original position.

Example 3: No Padding if Already Long Enough

text = "12345"
print(text.zfill(4))

12345

The string remains unchanged because its length is already more than the specified width.

Use Cases

• Formatting IDs with fixed length (e.g., invoice IDs like INV000123)
• Preparing data for export to systems that require zero-padded numbers
• Display time values like 05 instead of 5

Common Mistakes

• Using zfill() on non-string types like integers will raise an error. Convert to string first.
• Confusing zfill() with rjust() or ljust() which pad with spaces by default.

Interview Tip

zfill() is handy in problems requiring fixed-width formatting or normalization of numeric string data before comparison.

Summary

• zfill() pads the string on the left with 0s until it reaches the specified length.
• It preserves the sign position for negative numbers.
• Only works on string types — use str() on numbers before calling zfill().

Practice Problem

Write a program that asks the user for a number and prints it as a 6-digit zero-padded string.

user_input = input("Enter a number: ")
print("Padded Number:", user_input.zfill(6))