5. Collections.md

Python collections

Containers

• This lecture could also be named containers.

• Because containers are used to store things.

Programming - Big picture

• Programming is all about:
  • Storing data
  • Processing data
  • Storing processed data

Python collections

• List

• Dictionary

• Tuple

• String

• Set

• Frozenset

Collections hack :)

• Order matters everywhere except in dictionaries.

• “Dictionaries are order agnostic.”

Mutable/immutable

• Two types (dimensions) of mutability:

  • Can we add/delete elements from collection (box)
  • Can we change/update existing elements from collection (box)

• Mutable objects

  • List
  • Dictionary
  • Set
  • Bytearray

• Immutable objects

  • Integer
  • Float
  • String
  • Tuple
  • Frozen set
  • Complex
  • Bytes

1. Lists

• What can we do with list?

• What concepts are related to list? In which way?

• Computer can do only 3 things:

  • Perform some calculation
  • Remember what is calculated
  • Repeat process large number of time

• List is collection of elements.

• Those elements can be of different type.

Lists intro

• List indexing

  • We can access elements of list by using corresponding indices.

• Index starts from 0 !!!

• List slicing

  • We can get several elements of list using slicing operator.

• Two rules:

  • Including first
  • Excluding last.

Lists intro - list methods

• .copy
• .count
• .append
• .index
• .extend
• .clear
• .insert
• .pop
• .remove
• .reverse
• .sort
• ._ len _

List is a sequence

• List is just box which stores sequence of elements.

• Each element has its own unique index.

Lists are mutable

Traversing a list

• It is done using loops.

• Nested lists count as single element.

• Skip for now.

List operations

• “+” operator concatenates two lists.

• “ * ” operator repeats a list given number of times.

List slices

• my_list [x:y] - all elements from Xth (including) up to but not including Yth element.
• If you omit first element, it starts from beginning and goes up to Y-th element
• If you omit last element, it starts from X-th element (including) and goes towards end.

List methods

• Append
• Extend
• Sort
• …

Map, filter, reduce

• Very important concepts!

• Skip for now. When we learn loops, we will come back.

Deleting elements

• .pop
  • Delete and return n-th element in list
  • If n is not specified, then removes last element in list
• .remove
  • If you know element, but not its index
  • Deletes first element it sees
• del

List and strings

Objects and values

• Equivalent: two objects have same values

• Identical: if two objects are same i.e. two variables are pointing to same object.

• Objects are used for storing values.

• Objects can be equivalent, but not identical.

• Every identical object is also equivalent.

Aliasing (nickname)

• Association of variable with object is called reference.

• An object with mor ethan one reference has more than one name. This object is aliased.

• If aliased object is mutable, changes made with obe object affect other object.

• Aliasing is useful, but it is error prone.

• When working with mutable objects, it is best to avoid aliasing at all.

• Rule of thumb:

  • If you have return value, then modification is not performed in place.
  • If you have return value, then modification is done of ‘pass by value’ type.

Glossary

• List

• Element (aka item)

• Nested list

• Accumulator (variable i)

• Augmented assignment

• Reduce

• Map

• Filter

• Object

• Equivalent

• Identical

• Reference

• Aliasing

2. Dictionaries

• Collection of key-value pairs.

• Key must be unique, value does not have to be.

• Unordered collection of key-value pairs (aka items).

• List has index, dictionary has keys !!!

• Dictionary methods:

  • .copy (shallow copy)
  • .clear
  • .items
  • .keys
  • .values
  • .pop
  • .popitem
  • .update
  • ._ len _

A dictionary is a mapping

• key:value pairs

Looping and dictionaries

Dictionaries VS lists

Glossary

• Mapping

• Dictionary

• Key-value pairs

• Item

• Key

• Value

• Implementation

• Hashtable

• Hash function

• Hashtable

3. Tuples

• Immutable data type

• Collection of elements

• Tuple methods:

  • .index - returns index of first occurrence of value
  • .count - count how many particular elements exist in list

• Tuple operations:

  • Slicing
  • Multiplication
  • Relational operators (performed on element by element basis)

• Tuple creation:

  • A = 1,2,3,4
  • A = (1,2,3,4)

• Typical usage:

  • Multiple return values in function !!!

Iterators

• Iterators are collections similar to lists.
• You can iterate through.
• But, you can’t use index to access elements of iterator.

Zip and Enumerate

• Special and very useful functions.

Glossary

• Tuple
• Tuple assignment
• Zip object
• Iterator
• Data structure

4. Strings

• String methods:
  • .count
  • .index
  • .format
  • .capitalize
  • .casefold
  • .endswith
  • .expandtabs
  • .find
  • .join
  • .lower
  • .partition
  • .replace
  • .startswith
  • .strip

A string is a sequence

Traversal with a for loop

String slices

Strings are immutable

“in” operator

String comparison

Glossary

• Object
• Sequence
• Item
• Index
• Slice
• Empty string
• Immutable

5. Sets

• Mutable object i.e. can be changed.

• Data structure (collection) that can contains unique/distinct elements.

Set methods

• .add
• .clear
• .copy
• .difference
• .intersection
• …

6. Frozenset

• Immutable object i.e. cannot be changed.

• Can contain only unique/distinct elements.

Frozenset methods

• .copy

• .difference

• .intersection

• .isdisjoint

• .issubset

• .issuperset

• .symmetric_difference

• .union

The end

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