`str`: Alignment Methods¶

Python provides string alignment methods that pad text to a specified width. These methods are commonly used when formatting:

tables
reports
command-line output
logs

Alignment methods do not modify the original string. Instead, they return a new padded string.

s = "Hello"
t = s.ljust(10)

print(s)  # 'Hello'
print(t)  # 'Hello     '

Basic Alignment Methods¶

Python provides three primary alignment methods.

Method	Description
`ljust(width, fillchar)`	Left-align text
`rjust(width, fillchar)`	Right-align text
`center(width, fillchar)`	Center text

If the width is larger than the string length, padding is added.

Left Alignment¶

ljust() pads characters on the right side.

s = "Hello"

print(s.ljust(10))       # 'Hello     '
print(s.ljust(10, "-"))  # 'Hello-----'

Right Alignment¶

rjust() pads characters on the left side.

s = "Hello"

print(s.rjust(10))       # '     Hello'
print(s.rjust(10, "-"))  # '-----Hello'

Center Alignment¶

center() pads characters on both sides.

s = "Hello"

print(s.center(11))       # '   Hello   '
print(s.center(11, "-"))  # '---Hello---'

If padding is uneven, the extra space is placed on the right side.

print("Hello".center(10))  # '  Hello   '

Width Rules¶

Alignment methods never truncate strings.

If the specified width is smaller than the string length, the original string is returned unchanged.

s = "Hello World"

print(s.ljust(5))   # 'Hello World'
print(s.rjust(5))   # 'Hello World'
print(s.center(5))  # 'Hello World'

Zero Padding with `zfill()`¶

zfill() pads a string with leading zeros. It is specialized for numeric padding.

s = "42"

print(s.zfill(5))   # '00042'
print(s.zfill(10))  # '0000000042'

Unlike rjust(), zfill() handles numeric signs correctly.

print("-42".zfill(5))  # '-0042'
print("+42".zfill(6))  # '+00042'

Practical Examples¶

Table Formatting¶

Alignment methods are useful for formatting columns.

def main():
    data = [
        ("Alice", 95),
        ("Bob", 87),
        ("Carol", 100)
    ]

    for name, score in data:
        print(name.ljust(10) + str(score).rjust(5))

if __name__ == "__main__":
    main()

Output:

Alice         95
Bob           87
Carol        100

File Numbering¶

Padding ensures consistent filename ordering.

def main():
    files = ["report", "data", "summary"]

    for i, name in enumerate(files, 1):
        filename = f"{str(i).zfill(3)}_{name}.txt"
        print(filename)

if __name__ == "__main__":
    main()

Output:

001_report.txt
002_data.txt
003_summary.txt

Time Formatting¶

Zero padding is often used when formatting time values.

def main():
    hours, minutes, seconds = 9, 5, 3

    time_str = (
        str(hours).zfill(2) + ":" +
        str(minutes).zfill(2) + ":" +
        str(seconds).zfill(2)
    )

    print(time_str)  # 09:05:03

if __name__ == "__main__":
    main()

Alignment vs Format Specifiers¶

Alignment can also be performed using format specifiers in f-strings.

s = "Hi"
width = 10

print(s.ljust(width))    # 'Hi        '
print(s.rjust(width))    # '        Hi'
print(s.center(width))   # '    Hi    '

print(f"{s:<{width}}")   # 'Hi        '
print(f"{s:>{width}}")   # '        Hi'
print(f"{s:^{width}}")   # '    Hi    '

Custom fill characters are also supported.

print("Hi".center(10, "*"))  # ****Hi****
print(f"{'Hi':*^10}")        # ****Hi****

When to Use Each¶

Approach	Best Use
Alignment methods	when width is stored in variables
Format specifiers	when formatting output in f-strings

Key Takeaways¶

ljust(), rjust(), and center() align strings within a specified width.
Alignment methods return new strings and never modify the original.
If the width is smaller than the string length, the string is returned unchanged.
zfill() pads numbers with leading zeros while preserving signs.
Alignment is commonly used for tables, filenames, and formatted output.
Format specifiers provide an alternative alignment method in f-strings.

str: Alignment Methods¶