String formatting / padding

Ensure a string has a minimum width by adding leading zeros, commonly for numeric identifiers.

Formatting IDs or codes for display, preparing fixed-width file fields, generating zero-padded numbers for timestamps.

Data processing scripts, log formatting, generating serial numbers, preparing inputs for fixed-width file formats.

string_var.zfill(width)

... .zfill(...)

string_var: str, width: int

• str.rjust
• str.ljust
• str.center
• f-string formatting
• format()

• f'{string_var:0>{width}}'
• string_var.rjust(width
• '0')

Applying zfill to non‑string objects (misconception: assumes it works on any type) → TypeError or silent failure; expecting it to pad with spaces (misconception: confusing with rjust/ljust) → output lacks expected width; assuming it modifies the original string in place (misconception: mutability) → original string unchanged, leading to bugs.

rjust(width, '0') – achieves same zero‑padding but is more general; ljust – pads on the right side.

Coming from C: learners may expect zfill to accept integer arguments directly → they must convert numbers to strings first; Coming from C: learners may expect zfill to pad with other characters → zfill only pads with '0', use rjust/ljust or format for other fill chars.

• str.rjust
• str.ljust
• str.center

zfill only works on strings; if the string starts with a sign (+/-) the sign is kept and zeros are added after it. If the requested width is less than the string’s length, the original string is returned unchanged.

Guarantees uniform width for numeric fields, preventing misalignment in fixed‑width output and making sorting lexicographically equivalent to numeric order.

Think “zero fill” to pad numbers with leading zeros.

zfill returns a new string and does not modify the original; it only accepts str objects, so non‑string inputs must be converted first

Use f'{num:0>{width}}' or format(num, '0{}d'.format(width)) for more flexible padding (e.g., different fill characters).