In Python when to use a Dictionary List or Set

Question

When should I use a dictionary  list or set   Are there scenarios that are more suited for each data type

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When you want an unordered collection of unique elements, use a set. (For example, when you want the set of all the words used in a document).

When you want to collect an immutable ordered list of elements, use a tuple. (For example, when you want a (name, phone_number) pair that you wish to use as an element in a set, you would need a tuple rather than a list since sets require elements be immutable).

When you want to collect a mutable ordered list of elements, use a list. (For example, when you want to append new phone numbers to a list: [number1, number2, ...]).

When you want a mapping from keys to values, use a dict. (For example, when you want a telephone book which maps names to phone numbers: {'John Smith' : '555-1212'}). Note the keys in a dict are unordered. (If you iterate through a dict (telephone book), the keys (names) may show up in any order).

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Although this doesn t cover sets  it is a good explanation of dicts and lists      Lists are what they seem - a list of values  Each one of them is   numbered  starting from zero - the first one is numbered zero  the   second 1  the third 2  etc  You can remove values from the list  and   add new values to the end  Example  Your many cats  names       Dictionaries are similar to what their name suggests - a dictionary    In a dictionary  you have an  index  of words  and for each of them a   definition  In python  the word is called a  key   and the definition   a  value   The values in a dictionary aren t numbered - tare similar   to what their name suggests - a dictionary  In a dictionary  you have   an  index  of words  and for each of them a definition  The values in   a dictionary aren t numbered - they aren t in any specific order    either - the key does the same thing  You can add  remove  and modify   the values in dictionaries  Example  telephone book    http   www sthurlow com python lesson06

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For C   I was always having this flow chart in mind  In which scenario do I use a particular STL container   so I was curious if something similar is available for Python3 as well  but I had no luck   What you need to keep in mind for Python is  There is no single Python standard as for C    Hence there might be huge differences for different Python interpreters  e g  CPython  PyPy   The following flow chart is for CPython   Additionally I found no good way to incorporate the following data structures into the diagram  bytes  byte arrays  tuples  named tuples  ChainMap  Counter  and arrays    OrderedDict and deque are available via collections module  heapq is available from the heapq module LifoQueue  Queue  and PriorityQueue are available via the queue module which is designed for concurrent  threads  access   There is also a multiprocessing Queue available but I don t know the differences to queue Queue but would assume that it should be used when concurrent access from processes is needed   dict  set  frozen set  and list are builtin of course   For anyone I would be grateful if you could improve this answer and provide a better diagram in every aspect  Feel free and welcome    PS  the diagram has been made with yed  The graphml file is here

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When use them  I make an exhaustive cheatsheet of their methods for your reference   class ContainerMethods      def   init   self           self list methods 11                          Add    append   extend   insert                         Subtract    pop   remove                         Sort    reverse    sort                         Search    count    index                         Entire    clear   copy                                          self tuple methods 2     Search   count   index            self dict methods 11                          Views    keys    values    items                         Add    update                         Subtract    pop    popitem                          Extract    get   setdefault                          Entire     clear    copy   fromkeys                                          self set methods 17                         Add     add    update     difference update   symmetric difference update   intersection update                          Subtract    pop    remove   discard                         Relation    isdisjoint    issubset    issuperset                         operation    union   intersection   difference    symmetric difference                        Entire    clear    copy

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Do you just need an ordered sequence of items  Go for a list  Do you just need to know whether or not you ve already got a particular value  but without ordering  and you don t need to store duplicates   Use a set  Do you need to associate values with keys  so you can look them up efficiently  by key  later on  Use a dictionary

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A list keeps order  dict and set don t  when you care about order  therefore  you must use list  if your choice of containers is limited to these three  of course -    dict associates with each key a value  while list and set just contain values  very different use cases  obviously   set requires items to be hashable  list doesn t  if you have non-hashable items  therefore  you cannot use set and must instead use list   set forbids duplicates  list does not  also a crucial distinction    A  multiset   which maps duplicates into a different count for items present more than once  can be found in collections Counter -- you could build one as a dict  if for some weird reason you couldn t import collections  or  in pre-2 7 Python as a collections defaultdict int   using the items as keys and the associated value as the count    Checking for membership of a value in a set  or dict  for keys  is blazingly fast  taking about a constant  short time   while in a list it takes time proportional to the list s length in the average and worst cases   So  if you have hashable items  don t care either way about order or duplicates  and want speedy membership checking  set is better than list

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Dictionary  A python dictionary is used like a hash table with key as index and object as value   List  A list is used for holding objects in an array indexed by position of that object in the array   Set  A set is a collection with functions that can tell if an object is present or not present in the set

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In combination with lists  dicts and sets  there are also another interesting python objects  OrderedDicts       Ordered dictionaries are just like regular dictionaries but they remember the order that items were inserted  When iterating over an ordered dictionary  the items are returned in the order their keys were first added    OrderedDicts could be useful when you need to preserve the order of the keys  for example working with documents  It s common to need the vector representation of all terms in a document  So using OrderedDicts you can efficiently verify if a term has been read before  add terms  extract terms  and after all the manipulations you can extract the ordered vector representation of them

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Lists are what they seem - a list of values  Each one of them is numbered  starting from zero - the first one is numbered zero  the second 1  the third 2  etc  You can remove values from the list  and add new values to the end  Example  Your many cats  names   Tuples are just like lists  but you can t change their values  The values that you give it first up  are the values that you are stuck with for the rest of the program  Again  each value is numbered starting from zero  for easy reference  Example  the names of the months of the year   Dictionaries are similar to what their name suggests - a dictionary  In a dictionary  you have an  index  of words  and for each of them a definition  In python  the word is called a  key   and the definition a  value   The values in a dictionary aren t numbered - tare similar to what their name suggests - a dictionary  In a dictionary  you have an  index  of words  and for each of them a definition  In python  the word is called a  key   and the definition a  value   The values in a dictionary aren t numbered - they aren t in any specific order  either - the key does the same thing  You can add  remove  and modify the values in dictionaries  Example  telephone book

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In short  use   list - if you require an ordered sequence of items   dict - if you require to relate values with keys  set - if you require to keep unique elements   Detailed Explanation  List  A list is a mutable sequence  typically used to store collections of homogeneous items   A list implements all of the common sequence operations    x in l and x not in l l i   l i j   l i j k  len l   min l   max l  l count x  l index x   i   j    - index of the 1st occurrence of x in l  at or after i and before j indeces    A list also implements all of the mutable sequence operations    l i    x - item i of l is replaced by x l i j    t - slice of l from i to j is replaced by the contents of the iterable t del l i j  - same as l i j       l i j k    t - the elements of l i j k  are replaced by those of t del l i j k  - removes the elements of s i j k  from the list l append x  - appends x to the end of the sequence  l clear   - removes all items from l  same as del l     l copy   - creates a shallow copy of l  same as l     l extend t  or l    t - extends l with the contents of t l    n - updates l with its contents repeated n times l insert i  x  - inserts x into l at the index given by i l pop  i   - retrieves the item at i and also removes it from l l remove x  - remove the first item from l where l i  is equal to x l reverse   - reverses the items of l in place   A list could be used as stack by taking advantage of the methods append and pop   Dictionary  A dictionary maps hashable values to arbitrary objects  A dictionary is a mutable object  The main operations on a dictionary are storing a value with some key and extracting the value given the key   In a dictionary  you cannot use as keys values that are not hashable  that is  values containing lists  dictionaries or other mutable types    Set  A set is an unordered collection of distinct hashable objects  A set is commonly used to include membership testing  removing duplicates from a sequence  and computing mathematical operations such as intersection  union  difference  and symmetric difference

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Use a dictionary when you have a set of unique keys that map to values  Use a list if you have an ordered collection of items  Use a set to store an unordered set of items

[python] In Python, when to use a Dictionary, List or Set?

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