PyInfo
Python Fundamentals contents

Strings

Indexing, slicing, the most useful string methods, escape sequences and f-strings.

Beginner3 min readstringsslicingmethods

Read first: Variables and Data Types

A string is an immutable sequence of characters. Single, double and triple quotes all create one:

python
a = 'single'
b = "double: it's fine"
c = """triple quotes
can span several lines"""
assert len(a) == 6

Indexing

Characters are numbered from 0. Negative indices count from the end.

python
s = "PyInfo"
assert s[0] == "P" and s[2] == "I"
assert s[-1] == "o" and s[-2] == "f"
assert len(s) == 6

Strings are immutable: s[0] = "p" raises a TypeError. To "change" a string, build a new one.

Slicing

s[start:stop:step] takes characters from start up to but not including stop. Any part may be omitted.

python
s = "abcdefgh"
assert s[2:5] == "cde"        # indexes 2, 3, 4
assert s[:3] == "abc"         # from the beginning
assert s[5:] == "fgh"         # to the end
assert s[-3:] == "fgh"        # the last three
assert s[::2] == "aceg"       # every second character
assert s[::-1] == "hgfedcba"  # reversed
assert s[6:100] == "gh"       # out-of-range slices are clipped, not errors

s[::-1] is the idiomatic way to reverse a string, and the core of the palindrome check:

python
def is_palindrome(text):
    return text == text[::-1]

assert is_palindrome("level") and not is_palindrome("python")

Useful methods

Strings never change in place; every method returns a new string.

python
s = "  Hello, World  "
assert s.strip() == "Hello, World"                 # trim both ends (lstrip/rstrip for one side)
assert "abc".upper() == "ABC" and "ABC".lower() == "abc"
assert "hello world".title() == "Hello World"
assert "hello".capitalize() == "Hello"
assert "banana".count("an") == 2                    # non-overlapping occurrences
assert "banana".find("na") == 2                     # -1 if absent
assert "banana".index("na") == 2                    # ValueError if absent
assert "banana".replace("a", "o") == "bonono"
assert "a,b,c".split(",") == ["a", "b", "c"]
assert " ".join(["a", "b", "c"]) == "a b c"
assert "Python".startswith("Py") and "Python".endswith("on")
assert "123".isdigit() and "abc".isalpha() and "ab1".isalnum()

"sep".join(list) is the inverse of split and is far faster than adding strings in a loop.

Membership is tested with in:

python
assert "nan" in "banana"
assert "x" not in "banana"

Iterating over a string

python
vowels = 0
for ch in "programming":
    if ch in "aeiou":
        vowels += 1
assert vowels == 3

ord(ch) gives the character code and chr(code) the reverse, which is handy for shifting letters:

python
assert ord("a") == 97 and chr(98) == "b"
assert chr(ord("a") + 3) == "d"      # Caesar shift by 3

Escape sequences

A backslash introduces a special character:

Sequence Meaning
\n newline
\t tab
\\ a backslash
\' \" a quote

Prefix a string with r to disable escapes (r"C:\new"), which is useful for paths and regular expressions.

f-strings and formatting

python
name, score = "Ana", 9.5
assert f"{name} scored {score}" == "Ana scored 9.5"
assert f"{score:6.2f}" == "  9.50"      # width 6, 2 decimals
assert f"{255:b}" == "11111111"          # binary
assert f"{255:x}" == "ff"               # hexadecimal
assert f"{1234567:,}" == "1,234,567"
assert f"{name!r}" == "'Ana'"

Exercises

  1. Read a name and print it reversed.
  2. Count the vowels in a line of text (case-insensitive).
  3. Given a string, print it with the first and last character swapped.
  4. Check whether two words are anagrams (hint: compare sorted(a) and sorted(b)).
  5. Encode a message with a Caesar cipher of shift kk.

Practice

Apply this on the platform and get an instant verdict.

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