How do I get the number of elements in a list

Question

Consider the following   items      items append  apple   items append  orange   items append  banana      FAKE METHOD  items amount      Should return 3   How do I get the number of elements in the list items

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Answering your question as the examples also given previously   items      items append  apple   items append  orange   items append  banana    print items   len

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How to get the size of a list   To find the size of a list  use the builtin function  len  items      items append  quot apple quot   items append  quot orange quot   items append  quot banana quot    And now  len items   returns 3  Explanation Everything in Python is an object  including lists  All objects have a header of some sort in the C implementation  Lists and other similar builtin objects with a  quot size quot  in Python  in particular  have an attribute called ob size  where the number of elements in the object is cached  So checking the number of objects in a list is very fast  But if you re checking if list size is zero or not  don t use len - instead  put the list in a boolean context - it treated as False if empty  True otherwise  From the docs len s   Return the length  the number of items  of an object  The argument may be a sequence  such as a string  bytes  tuple  list  or range  or a collection  such as a dictionary  set  or frozen set    len is implemented with   len    from the data model docs  object   len   self   Called to implement the built-in function len    Should return the length of the object  an integer  gt   0  Also  an object that doesn   t define a   nonzero      in Python 2 or   bool     in Python 3  method and whose   len     method returns zero is considered to be false in a Boolean context   And we can also see that   len   is a method of lists  items   len      returns 3  Builtin types you can get the len  length  of And in fact we see we can get this information for all of the described types   gt  gt  gt  all hasattr cls     len     for cls in  str  bytes  tuple  list                                               range  dict  set  frozenset   True  Do not use len to test for an empty or nonempty list To test for a specific length  of course  simply test for equality  if len items     required length           But there s a special case for testing for a zero length list or the inverse  In that case  do not test for equality  Also  do not do  if len items             Instead  simply do  if items        Then we have some items  not empty           or if not items    Then we have an empty list           I explain why here but in short  if items or if not items is both more readable and more performant

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While this may not be useful due to the fact that it d make a lot more sense as being  out of the box  functionality  a fairly simple hack would be to build a class with a length property   class slist list        property     def length self           return len self    You can use it like so    gt  gt  gt  l   slist range 10    gt  gt  gt  l length 10  gt  gt  gt  print l  0  1  2  3  4  5  6  7  8  9    Essentially  it s exactly identical to a list object  with the added benefit of having an OOP-friendly length property   As always  your mileage may vary

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Besides len you can also use operator length hint  requires Python 3 4    For a normal list both are equivalent  but length hint makes it possible to get the length of a list-iterator  which could be useful in certain circumstances    gt  gt  gt  from operator import length hint  gt  gt  gt  l     apple    orange    banana    gt  gt  gt  len l  3  gt  gt  gt  length hint l  3   gt  gt  gt  list iterator   iter l   gt  gt  gt  len list iterator  TypeError  object of type  list iterator  has no len    gt  gt  gt  length hint list iterator  3   But length hint is by definition only a  hint   so most of the time len is better   I ve seen several answers suggesting accessing   len    This is all right when dealing with built-in classes like list  but it could lead to problems with custom classes  because len  and length hint  implement some safety checks  For example  both do not allow negative lengths or lengths that exceed a certain value  the sys maxsize value   So it s always safer to use the len function instead of the   len   method

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There is an inbuilt function called len   in python which will help in these conditions  a  1 2 3 4 5 6  print len a        Here the len   function counts the number of items in the list   Output   gt  gt  gt  6  This will work slightly different in the case of string  below   a  quot Hello quot  print len a    Here the len   function counts the alphabets or characters in the list   Output   gt  gt  gt  5  This is because variable  a  is a string and not a list  so it will count the number of characters or alphabets in the string and then print the output

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To get the number of element in any iterable object  your goto method in Python is  len   eg  a   range 1000    range b    abcdefghijklmnopqrstuvwxyz    string c    10  20  30    List d    30  40  50  60  70    tuple e    11  21  31  41    set  len   method can work on all the above data types because they are iterable i e You can iterate over them  all var    a  b  c  d  e    All variables are stored to a list for var in all var      print len var    Rough estimate of the len   method def len iterable          total   0     for i in iterable          total    1     return total

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The len   function can be used with several different types in Python - both built-in types and library types  For example   gt  gt  gt  len  1  2  3   3

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In terms of how len   actually works  this is its C implementation  static PyObject   builtin len PyObject  module  PyObject  obj     clinic end generated code  output fa7a270d314dfb6c input bc55598da9e9c9b5          Py ssize t res       res   PyObject Size obj       if  res  lt  0            assert PyErr Occurred             return NULL            return PyLong FromSsize t res      Py ssize t is the maximum length that the object can have  PyObject Size   is a function that returns the size of an object  If it cannot determine the size of an object  it returns -1  In that case  this code block will be executed      if  res  lt  0            assert PyErr Occurred             return NULL         And an exception is raised as a result  Otherwise  this code block will be executed      return PyLong FromSsize t res    res which is a C integer  is converted into a Python int  which is still called a  quot Long quot  in the C code because Python 2 had two types for storing integers  and returned

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And for completeness  primarily educational   it is possible without using the len   function  I would not condone this as a good option DO NOT PROGRAM LIKE THIS IN PYTHON  but it serves a purpose for learning algorithms   def count list       item count   0     for item in list             item count    1     return item count  count  1 2 3 4 5      The colon in list    is implicit and is therefore also optional    The lesson here for new programmers is  You can   t get the number of items in a list without counting them at some point  The question becomes  when is a good time to count them  For example  high-performance code like the connect system call for sockets  written in C  connect int sockfd  const struct sockaddr  addr  socklen t addrlen    does not calculate the length of elements  giving that responsibility to the calling code   Notice that the length of the address is passed along to save the step of counting the length first  Another option  computationally  it might make sense to keep track of the number of items as you add them within the object that you pass  Mind that this takes up more space in memory  See Naftuli Kay   s answer   Example of keeping track of the length to improve performance while taking up more space in memory  Note that I never use the len   function because the length is tracked   class MyList object       def   init   self           self  data              self length   0   length tracker that takes up memory but makes length op O 1  time             the implicit iterator in a list class     def   iter   self           for elem in self  data              yield elem      def add self  elem           self  data append elem          self length    1      def remove self  elem           self  data remove elem          self length -  1  mylist   MyList   mylist add 1  mylist add 2  mylist add 3  print mylist length    3 mylist remove 3  print mylist length    2

[python] How do I get the number of elements in a list?

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