Modify Strings

Every change makes a new string

Say it once more, because it explains every method on this page: strings are immutable. A "modifying" method returns a new string and leaves the original where it was.

name = "  Ada Lovelace  "
print(repr(name.strip()))
print(repr(name))

'Ada Lovelace'
'  Ada Lovelace  '

repr() shows the quotes and escapes, which is how you can see the spaces at all.

Case

name = "Ada Lovelace"
print(name.upper())
print(name.lower())
print(name.title())
print("hELLO".capitalize())

ADA LOVELACE
ada lovelace
Ada Lovelace
Hello

capitalize() uppercases the first character and lowercases everything else, which is not the same as title().

Trimming whitespace

padded = "\t  hello \n"
print(repr(padded.strip()))
print(repr(padded.lstrip()))
print(repr(padded.rstrip()))

'hello'
'hello \n'
'\t  hello'

With an argument, strip() removes any of those characters from the ends — it is a set of characters, not a prefix:

print("xxhelloxx".strip("x"))
print("www.example.com".strip("wmoc."))

hello
example

print("www.example.com".removeprefix("www."))
print("report.pdf".removesuffix(".pdf"))

example.com
report

Replacing

line = "one,two,three"
print(line.replace(",", " | "))
print("aaa".replace("a", "b", 2))

one | two | three
bba

The third argument caps how many replacements happen.

Splitting and joining

split() breaks a string into a list; join() puts one back together:

line = "one,two,three"
parts = line.split(",")
print(parts)
print(" and ".join(parts))

['one', 'two', 'three']
one and two and three

The separator goes between items, so three items produce two separators. Note which side it is written on: the separator is the string, and the list is the argument. ",".join(parts) reads backwards the first few times and then never again.

With no argument, split() breaks on any run of whitespace and drops the empty pieces, which is almost always what you want for typed input:

print("  a   b \n c ".split())
print("  a   b \n c ".split(" "))

['a', 'b', 'c']
['', '', 'a', '', '', 'b', '\n', 'c', '']

Building a string in a loop

Adding to a string in a loop makes a new string every time. For a few items nobody notices; for many, collect them and join once:

words = ["fast", "and", "readable"]
sentence = " ".join(words)
print(sentence)

fast and readable