Python Strings: Slicing, Methods and f-Strings

Creating strings

A string is a sequence of Unicode characters. Single and double quotes are interchangeable; triple quotes span multiple lines:

a = 'single'
b = "double"
c = """This string
spans lines"""
d = "She said \"hi\""     # escape a quote with backslash
e = 'It\'s fine'
print(a, b, d, e, sep=" | ")
print(c)

Useful escape sequences: \n newline, \t tab, \\ backslash. Prefix with r for a raw string where backslashes are literal — handy for file paths and regular expressions: r"C:\new\folder".

Indexing and slicing

Every character has a position starting at 0. Negative positions count from the end. A slice s[start:stop:step] takes characters from start up to but not including stop:

s = "Python"
print(s[0], s[-1])      # first, last
print(s[0:3])           # Pyt
print(s[2:])            # thon
print(s[:2])            # Py
print(s[::2])           # Pto (every 2nd)
print(s[::-1])          # nohtyP (reversed)
print(len(s))

Strings are immutable — s[0] = "J" raises TypeError. Build a new string instead: "J" + s[1:].

The string methods you will actually use

t = "  Hello, World  "
print(t.strip())               # remove surrounding whitespace
print(t.lower(), t.upper())
print(t.strip().title())
print("a,b,c".split(","))        # -> list
print("-".join(["x", "y", "z"]))  # list -> string
print("banana".count("a"))
print("banana".replace("a", "o"))
print("banana".find("nan"))      # index or -1
print("hello".startswith("he"), "hello".endswith("lo"))
print("42".isdigit(), "abc".isalpha())
print("py".center(10, "*"))

Methods never change the original string; they return a new one. t.strip() on its own does nothing unless you keep the result: t = t.strip().

f-strings: the modern way to format

Since Python 3.6, f-strings are the clearest way to build text from values. Put f before the quote and expressions in braces:

name, score, pi = "Ada", 93.4567, 3.14159
print(f"{name} scored {score}")
print(f"{score:.1f}")          # 1 decimal place
print(f"{pi:.3f}")
print(f"{1234567:,}")          # thousands separators
print(f"{0.256:.0%}")          # percentage
print(f"{name:>10}|{name:<10}|{name:^10}|")   # alignment
print(f"{2 ** 10 = }")         # debug form (3.8+)

Older code uses "{} {}".format(a, b) or "%s %d" % (a, b). Both still work, but prefer f-strings for anything new.

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Checking membership and comparing

email = "ada@example.com"
print("@" in email)
print("gmail" not in email)
print("apple" < "banana")     # alphabetical
print("Apple" == "apple", "Apple".lower() == "apple")

Common mistakes

• Adding a number to a string. "Age: " + 27 raises TypeError. Use f"Age: {27}" or str(27).
• Forgetting methods return new strings. Assign the result.
• Off-by-one slices. The stop index is exclusive. s[0:3] is three characters, not four.
• Comparing with different cases. Normalise with .lower() or .casefold() before comparing user input.

Next: numbers and operators.

Frequently asked questions