String Methods

The methods you will actually reach for, grouped by what you want done.

Asking questions

These return True or False and are the backbone of input checking:

Python

name = "report.pdf"
print(name.startswith("rep"))
print(name.endswith(".pdf"))
print("abc123".isalnum())
print("123".isdigit())
print("abc".isalpha())
print("   ".isspace())

Output

True
True
True
True
True
True

startswith and endswith accept a tuple, which beats a chain of or:

Python

for name in ["a.png", "b.txt", "c.jpg"]:
    print(name, name.endswith((".png", ".jpg")))

Output

a.png True
b.txt False
c.jpg True

Python

print("-4".isdigit())
print("3.5".isdigit())

Output

False
False

Finding

find() gives the index, or -1 when there is nothing there. index() does the same but raises instead, and count() tallies:

Python

line = "one,two,one"
print(line.find("two"))
print(line.find("three"))
print(line.count("one"))

Output

4
-1
2

Use in when you only want to know whether it is there, and find when you need to know where.

Changing case

Python

print("ada".upper())
print("ADA".lower())
print("ada lovelace".title())
print("Ada".swapcase())

Output

ADA
ada
Ada Lovelace
aDA

For case-insensitive comparison, casefold() is the thorough version of lower() and handles scripts where the two differ:

Python

print("Straße".casefold() == "strasse".casefold())
print("Straße".lower() == "strasse".lower())

Output

True
False

Splitting

Python

line = "a,b,c"
print(line.split(","))
print(line.rsplit(",", 1))
print("first line\nsecond".splitlines())
print("key=value=more".partition("="))

Output

['a', 'b', 'c']
['a,b', 'c']
['first line', 'second']
('key', '=', 'value=more')

partition() always gives exactly three pieces, which makes it safe to unpack without checking the length first.

Padding

Python

print("7".zfill(3))
print("ab".ljust(6, ".") + "|")
print("ab".rjust(6, ".") + "|")
print("ab".center(6, ".") + "|")

Output

007
ab....|
....ab|
..ab..|

A quick reference

Method	Does
`strip()`	remove whitespace from both ends
`removeprefix()`	drop an exact prefix
`replace(a, b)`	swap every `a` for `b`
`split(sep)`	break into a list
`join(items)`	glue a list together
`startswith(x)`	test the beginning
`find(x)`	index, or `-1`
`zfill(n)`	pad with leading zeros
`encode()`	turn into bytes

Every one of these returns something new. None of them change the string you called them on.

Test yourself

3 questions

What does "-4".isdigit() give?

Show the answer

False — The minus sign is not a digit. isdigit is not a number check; try converting and catch ValueError instead.

What does "one,two".find("three") give?

Show the answer

-1 — find returns -1 when there is no match. index() raises instead, and 'in' is best when you only need yes or no.

What does "key=value=more".partition("=") give?

Show the answer

('key', '=', 'value=more') — partition always gives exactly three pieces, so it is safe to unpack without checking the length.

Next chapter

Text and Unicode

What a character really is, and the difference between text and bytes.