How to declare an array in Python

Question

How do I declare an array in Python   I can t find any reference to arrays in the documentation

User · Answer

This is surprisingly complex topic in Python.

Practical answer

Arrays are represented by class list (see reference and do not mix them with generators).

Check out usage examples:

# empty array
arr = [] 

# init with values (can contain mixed types)
arr = [1, "eels"]

# get item by index (can be negative to access end of array)
arr = [1, 2, 3, 4, 5, 6]
arr[0]  # 1
arr[-1] # 6

# get length
length = len(arr)

# supports append and insert
arr.append(8)
arr.insert(6, 7)

Theoretical answer

Under the hood Python's list is a wrapper for a real array which contains references to items. Also, underlying array is created with some extra space.

Consequences of this are:

random access is really cheap (arr[6653] is same to arr[0])
append operation is 'for free' while some extra space
insert operation is expensive

Check this awesome table of operations complexity.

Also, please see this picture, where I've tried to show most important differences between array, array of references and linked list:

User · Answer

variable        Now variable refers to an empty list    Of course this is an assignment  not a declaration  There s no way to say in Python  this variable should never refer to anything other than a list   since Python is dynamically typed      The default built-in Python type is called a list  not an array  It is an ordered container of arbitrary length that can hold a heterogenous collection of objects  their types do not matter and can be freely mixed   This should not be confused with the array module  which offers a type closer to the C array type  the contents must be homogenous  all of the same type   but the length is still dynamic

User · Answer

To add to Lennart s answer  an array may be created like this   from array import array float array   array  f  values    where values can take the form of a tuple  list  or np array  but not array   values    1 2 3  values    1 2 3  values   np array  1 2 3   f      i  will work here too  but if array is  i  then values have to be int wrong values   array  f   1 2 3     TypeError   array array  object is not callable   and the output will still be the same   print float array  print float array 1   print isinstance float array 1  float      array  f    1 0  2 0  3 0     2 0   True   Most methods for list work with array as well  common ones being pop    extend    and append     Judging from the answers and comments  it appears that the array data structure isn t that popular  I like it though  the same way as one might prefer a tuple over a list   The array structure has stricter rules than a list or np array  and this can reduce errors and make debugging easier  especially when working with numerical data   Attempts to insert append a float to an int array will throw a TypeError   values    1 2 3  int array   array  i  values  int array append float 1     or int array extend  float 1       TypeError  integer argument expected  got float   Keeping values which are meant to be integers  e g  list of indices  in the array form may therefore prevent a  TypeError  list indices must be integers  not float   since arrays can be iterated over  similar to np array and lists   int array   array  i   1 2 3   data    11 22 33 44 55  sample      for i in int array      sample append data i     Annoyingly  appending an int to a float array will cause the int to become a float  without throwing an exception   np array retain the same data type for its entries too  but instead of giving an error it will change its data type to fit new entries  usually to double or str    import numpy as np numpy int array   np array  1 2 3   i   for i in numpy int array      print type i          lt class  numpy int32  gt  numpy int array 2   np append numpy int array int 1     still  lt class  numpy int32  gt  numpy float array   np append numpy int array float 1      lt class  numpy float64  gt  for all values numpy str array   np append numpy int array  1      lt class  numpy str   gt  for all values data    11 22 33 44 55  sample      for i in numpy int array 2      sample append data i         no problem here  but TypeError for the other two   This is true during assignment as well  If the data type is specified  np array will  wherever possible  transform the entries to that data type   int numpy array   np array  1 2 float 3    i     3 becomes an int int numpy array 2   np array  1 2 3 9   i     3 9 gets truncated to 3  same as int 3 9   invalid array   np array  1 2  string    i     ValueError  invalid literal for int   with base 10   string    Same error as int  string   str numpy array   np array  1 2 3   str   print str numpy array  print  type i  for i in str numpy array       1   2   3      lt class  numpy str   gt    or  in essence   data    1 2 3 4 5 6  list 1   np array data  i   tolist   list 2    int i  for i in data  print list 1    list 2    True   while array will simply give   invalid array   array  1 2 3 9   i     TypeError  integer argument expected  got float   Because of this  it is not a good idea to use np array for type-specific commands  The array structure is useful here  list preserves the data type of the values   And for something I find rather pesky  the data type is specified as the first argument in array    but  usually  the second in np array        The relation to C is referred to here  Python List vs  Array - when to use   Have fun exploring   Note  The typed and rather strict nature of array leans more towards C rather than Python  and by design Python does not have many type-specific constraints in its functions  Its unpopularity also creates a positive feedback in collaborative work  and replacing it mostly involves an additional  int x  for x in file   It is therefore entirely viable and reasonable to ignore the existence of array  It shouldn t hinder most of us in any way   D

User · Answer

This is how   my array    1   rebecca    allard   15

User · Answer

I think you  meant want an list with the first 30 cells already filled  So     f          for i in range 30          f append 0    An example to where this could be used is in Fibonacci sequence  See problem 2 in Project Euler

User · Answer

I had an array of strings and needed an array of the same length of booleans initiated to True  This is what I did  strs     Hi   Bye    bools     True for s in strs

User · Answer

A couple of contributions suggested that arrays in python are represented by lists  This is incorrect  Python has an independent implementation of array   in the standard library module array   array array    hence it is incorrect to confuse the two  Lists are lists in python so be careful with the nomenclature used    list 01    4  6 2  7-2j   flo    cro    list 01 Out 85    4  6 2   7-2j    flo    cro     There is one very important difference between list and array array     While both of these objects are ordered sequences  array array   is an ordered homogeneous sequences whereas a list is a non-homogeneous sequence

User · Answer

How about this      gt  gt  gt  a   range 12   gt  gt  gt  a  0  1  2  3  4  5  6  7  8  9  10  11   gt  gt  gt  a 7  6

User · Answer

Following on from Lennart  there s also numpy which implements homogeneous multi-dimensional arrays

User · Answer

JohnMachin s comment should be the real answer  All the other answers are just workarounds in my opinion  So   array  0  element count

User · Answer

You can create lists and convert them into arrays or you can create array using numpy module  Below are few examples to illustrate the same  Numpy also makes it easier to work with multi-dimensional arrays   import numpy as np a   np array  1  2  3  4     For custom inputs a   np array  int x  for x in input   split       You can also reshape this array into a 2X2 matrix using reshape function which takes in input as the dimensions of the matrix   mat   a reshape 2  2

User · Answer

For calculations  use numpy arrays like this   import numpy as np  a   np ones  3 2            a 2D array with 3 rows  2 columns  filled with ones b   np array  1 2 3         a 1D array initialised using a list  1 2 3  c   np linspace 2 3 100     an array with 100 points beteen  and including  2 and 3  print a 1 5     all elements of a times 1 5 print a T b     b added to the transpose of a   these numpy arrays can be saved and loaded from disk  even compressed  and complex calculations with large amounts of elements are C-like fast    Much used in scientific environments  See here for more

User · Answer

I would normally just do a    1 2 3  which is actually a list but for arrays look at this formal definition

User · Answer

You don t declare anything in Python  You just use it  I recommend you start out with something like http   diveintopython net

User · Answer

Python calls them lists  You can write a list literal with square brackets and commas    gt  gt  gt   6 28 496 8128   6  28  496  8128

User · Answer

You don t actually declare things  but this is how you create an array in Python   from array import array intarray   array  i     For more info see the array module  http   docs python org library array html  Now possible you don t want an array  but a list  but others have answered that already

[python] How to declare an array in Python?

Practical answer

Theoretical answer

Examples related to python

Examples related to arrays