Strings & Dictionaries

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String formatting

String concatenation

So far, we've been using the + operator for combining string literals with the results of expressions.


            artist = "Lil Nas X"
            song = "Industry Baby"
            place = 2

            print("Debuting at #" + str(place) + ": '" + song + "' by " + artist)

But that's not ideal:

Easy to bungle up the + signs
Hard to grok what the final string will be
Requires explicitly str()ing non-strings

String interpolation

String interpolation is the process of combining string literals with the results of expressions.

Available since Python 3.5, f strings (formatted string literals) are the best way to do string interpolation.

Just put an f in front of the quotes and then put any valid Python expression in curly brackets inside:


            artist = "Lil Nas X"
            song = "Industry Baby"
            place = 2

            print(f"Debuting at #{place}: '{song}' by {artist}")

😍😍😍😍😍😍

String operations

Strings as sequences

Strings and lists are both considered sequences in Python. so both support standard sequence operations, like +, *, in, and bracket notation.


          title = 'The Parable of the Sower'
          author = 'Octavia E. Butler'

          full = title + ' By ' + author
          contains_parable = 'Parable' in title
          chant = 'We want more ' + (author + ',') * 3 + '!'
          initials = author[0] + author[8] + author[11]
          title_length = len(title)

However: strings are immutable.
Once created, they can't be changed.

Bracket notation with strings

Bracket notation can be used to either access individual characters or to make "slices".


          adjective = 'superduper'

          adjective[0]   # 's'
          adjective[-1]  # 'r'

          adjective[0:5] # 'super'
          adjective[5:]  # 'duper'


          illness = 'pneumonoultramicroscopicsilicovolcanoconiosis'
          true_start_sound = illness[1]
          how_small = illness[13:24]
          from_where = illness[30:37]

Splitting strings

The string split() method turns a string into a list of strings, by splitting at the specified delimiter.


          my_groceries = 'apples,bananas,carrots'
          grocery_items = my_groceries.split(',')

          for item in grocery_items:
              print(item)

Python documentation: str.split

Joining strings

The string join() method returns a string which is the concatenation of the strings in an iterable.


          names = ["Gray", "Fox"]
          print("".join(names))

          address_parts = ["123 Pining St", "Nibbsville", "OH"]
          print(",".join(address_parts))

          poem_lines = ["Forgive me", "they were delicious", "so sweet", "and so cold"]
          print("\n".join(poem_lines))

Python documentation: str.join

More string methods

Python offers many string methods. A few more:

Method	Description
`str.replace(old, new)`	Returns a new string where occurrences of `old` are replaced with `new`.
`str.find(substring)`	Returns the lowest index in the string where `substring` is found.
`str.startswith(prefix)`	Returns `True` if the string starts with the `prefix`, and `False` otherwise.
`str.endswith(suffix)`	Returns `True` if the strings ends with the `prefix`, and `False` otherwise.
`str.strip([chars])`	Returns a copy of the string with leading and trailing characters removed, defaulting to whitespace characters.
`str.lower()`	Returns a copy of the string with all characters converted to lowercase.

Dictionaries

A dict is a mutable mapping of key-value pairs


            states = {
              "CA": "California",
              "DE": "Delaware",
              "NY": "New York",
              "TX": "Texas",
              "WY": "Wyoming"
            }

Queries:


            >>> len(states)
            5


            >>> "CA" in states
            True


            >>> "ZZ" in states
            False

Dictionary selection


            words = {
              "más": "more",
              "otro": "other",
              "agua": "water"
            }

Select a value:


            >>> words["otro"]
            'other'


            >>> first_word = "agua"
            >>> words[first_word]
            'water'


            >>> words["pavo"]
            KeyError: pavo


            >>> words.get("pavo", "🤔")
            '🤔'

Dictionary mutation

Create an empty dict:


            users = {}

Add values:


            users["profpamela"] = "b3stp@ssEvErDontHackMe"

Change values:


            users["profpamela"] += "itsLongerSoItsMoreSecure!!"


            >>> users["profpamela"]
            'b3stp@ssEvErDontHackMeitsLongerSoItsMoreSecure!!'

Dictionary rules

A key cannot be a list or dictionary (or any mutable type)
All keys in a dictionary are distinct (there can only be one value per key)
The values can be any type, however!


            spiders = {
              "smeringopus": {
                "name": "Pale Daddy Long-leg",
                "length": 7
              },
              "holocnemus pluchei": {
                "name": "Marbled cellar spider",
                "length": (5, 7)
              }
            }

Dictionary iteration


            insects = {"spiders": 8, "centipedes": 100, "bees": 6}
            for name in insects:
                print(insects[name])

...is the same as:


            for name in list(insects):
                print(insects[name])

What will be the order of items?


            8 100 6

Keys are iterated over in the order they are first added.