Python for Programmers: Strings as Collections

• Strings behave like lists in some ways. For example, we can index into them using the familiar indexing syntax.

• Unlike some programming languages, Python doesn't have a dedicated character data type. When we index into a string, we get another string containing only that character.

• >

  "cat"[1]

  Result:

  'a'

• This has one strange side effect. We can index into a string, giving us a single-character string. Then we can index into that single-character string at index 0, giving us the same single-character string again. Each time we index into the string, we get a string of length 1.

• >

  "cat"[1][0]

  Result:

  'a'

• >

  "cat"[2][0][0][0][0][0][0]

  Result:

  't'

• When we index outside of a string, Python raises an IndexError.

• >

  "cat"[3]

  Result:

  IndexError: string index out of range

• >

  "cat"[0]

  Result:

  'c'

• >

  "cat"[0][1]

  Result:

  IndexError: string index out of range

• Strings support slices, which we already saw for lists. In the next example, our string has a fixed format: "Ticket #" followed by a four-digit number. We use the slice syntax to extract just the number part.

• >

  ticket = "Ticket #1234"
  id = ticket[-4:]
  id

  Result:

  '1234'

• Slicing out of bounds gives us an empty string. This matches what we already saw for lists, where slicing out of bounds gives us an empty list.

• >

  data = "abc"
  data[10:15]

  Result:

  ''

• string1 in string2 tells us whether a string contains another string.

• >

  friends = "Betty, Cindy, Dalili"
• Note: this code example reuses elements (variables, etc.) defined in earlier examples.

  >

  "Betty" in friends

  Result:

  True
• Note: this code example reuses elements (variables, etc.) defined in earlier examples.

  >

  "Cindy" in friends

  Result:

  True
• Note: this code example reuses elements (variables, etc.) defined in earlier examples.

  >

  "Amir" in friends

  Result:

  False

• As usual, we can invert the in operator with string1 not in string2.

• Note: this code example reuses elements (variables, etc.) defined in earlier examples.

  >

  "Ebony" not in friends

  Result:

  True