Shuffling a list of objects

Question

I have a list of objects and I want to shuffle them  I thought I could use the random shuffle method  but this seems to fail when the list is of objects  Is there a method for shuffling objects or another way around this   import random  class A      foo    bar   a1   a   a2   a   b    a1  a2   print random shuffle b     This will fail

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Make sure you are not naming your source file random py  and that there is not a file in your working directory called random pyc    either could cause your program to try and import your local random py file instead of pythons random module

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import random  class a      foo    bar   a1   a   a2   a   a3   a   a4   a   b    a1 a2 a3 a4   random shuffle b  print b    shuffle  is in place  so do not print result  which is None  but the list

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random shuffle should work   Here s an example  where the objects are lists   from random import shuffle x     i  for i in range 10   shuffle x     print x   gives    9    2    7    0    4    5    3    1    8    6     of course your results will vary   Note that shuffle works in place  and returns None

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The shuffling process is  with replacement   so the occurrence of each item may change  At least when when items in your list is also list    E g    ml     0    1     10   After    random shuffle ml    The number of  0  may be 9 or 8  but not exactly 10

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As you learned the in-place shuffling was the problem  I also have problem frequently  and often seem to forget how to copy a list  too  Using sample a  len a   is the solution  using len a  as the sample size  See https   docs python org 3 6 library random html random sample for the Python documentation   Here s a simple version using random sample   that returns the shuffled result as a new list   import random  a   range 5  b   random sample a  len a   print a  b   two list same    a    b   print   0  1  2  3  4   2  1  3  4  0  two list same  False    The function sample allows no duplicates    Result can be smaller but not larger than the input  a   range 555  b   random sample a  len a   print  no duplicates    a    list set b    try      random sample a  len a    1  except ValueError as e      print  Nope    e    print  no duplicates  True   print  Nope  sample larger than population

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import random class a      foo    bar   a1   a   a2   a   b    a1 foo a2 foo  random shuffle b

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In some cases when using numpy arrays  using random shuffle created duplicate data in the array    An alternative is to use numpy random shuffle  If you re working with numpy already  this is the preferred method over the generic random shuffle    numpy random shuffle  Example   gt  gt  gt  import numpy as np  gt  gt  gt  import random   Using random shuffle    gt  gt  gt  foo   np array   1 2 3   4 5 6   7 8 9     gt  gt  gt  foo  array   1  2  3           4  5  6           7  8  9       gt  gt  gt  random shuffle foo   gt  gt  gt  foo  array   1  2  3           1  2  3           4  5  6      Using numpy random shuffle    gt  gt  gt  foo   np array   1 2 3   4 5 6   7 8 9     gt  gt  gt  foo  array   1  2  3           4  5  6           7  8  9       gt  gt  gt  np random shuffle foo   gt  gt  gt  foo  array   1  2  3           7  8  9           4  5  6

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print func foo   will print the return value of  func  when called with  foo    shuffle  however has None as its return type  as the list will be modified in place  hence it prints nothing  Workaround     shuffle the list in place  random shuffle b     print it print b    If you re more into functional programming style you might want to make the following wrapper function   def myshuffle ls       random shuffle ls      return ls

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from random import random my list   range 10  shuffled list   sorted my list  key lambda x  random      This alternative may be useful for some applications where you want to swap the ordering function

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It works fine  I am trying it here with functions as list objects       from random import shuffle      def foo1            print  foo1        def foo2            print  foo2        def foo3            print  foo3        A  foo1 foo2 foo3       for x in A          x        print   r       shuffle A      for y in A          y     It prints out  foo1 foo2 foo3  foo2 foo3 foo1  the foos in the last row have a random order

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It took me some time to get that too  But the documentation for shuffle is very clear      shuffle list x in place  return None    So you shouldn t print random shuffle b    Instead do random shuffle b  and then print b

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gt  gt  gt  import random  gt  gt  gt  a     hi   world   cat   dog    gt  gt  gt  random shuffle a random random   gt  gt  gt  a   hi    cat    dog    world     It works fine for me   Make sure to set the random method

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to shuffle random  set random  True      def shuffle x random False        shuffled           ma   x      if random    True           rando    ma i  for i in np random randint 0 len ma  len ma             return rando      if random    False            for i in range len ma              ave   len ma   3           if i  lt  ave               shuffled append ma i ave             else               shuffled append ma i-ave            return shuffled

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You can go for this    gt  gt  gt  A     r   a   n   d   o   m    gt  gt  gt  B    1 2 3 4 5 6   gt  gt  gt  import random  gt  gt  gt  random sample A B  len A B    3   r   4   n   6  5   m   2  1   a    o    d     if you want to go back to two lists  you then split this long list into two

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usr bin python3  import random  s list range 5   random shuffle s     lt  lt  shuffle before print or assignment print s     print   2  4  1  3  0

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If you happen to be using numpy already  very popular for scientific and financial applications  you can save yourself an import  import numpy as np     np random shuffle b  print b   https   numpy org doc stable reference random generated numpy random shuffle html

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you could build a function that takes a list as a parameter and returns a shuffled version of the list   from random import    def listshuffler inputlist       for i in range len inputlist            swap   randint 0 len inputlist -1          temp   inputlist swap          inputlist swap    inputlist i          inputlist i    temp     return inputlist

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you can either use shuffle or sample   both of which come from random module   import random def shuffle arr1       n len arr1      b random sample arr1 n      return b   OR  import random def shuffle arr1       random shuffle arr1      return arr1

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If you have multiple lists  you might want to define the permutation  the way you shuffle the list   rearrange the items in the list  first and then apply it to all lists   import random  perm   list range len list one    random shuffle perm  list one    list one index  for index in perm  list two    list two index  for index in perm    Numpy   Scipy  If your lists are numpy arrays  it is simpler   import numpy as np  perm   np random permutation len list one   list one   list one perm  list two   list two perm    mpu  I ve created the small utility package mpu which has the consistent shuffle function   import mpu    Necessary if you want consistent results import random random seed 8     Define example lists list one    1 2 3  list two     a    b    c      Call the function list one  list two   mpu consistent shuffle list one  list two    Note that mpu consistent shuffle takes an arbitrary number of arguments  So you can also shuffle three or more lists with it

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Plan  Write out the shuffle without relying on a library to do the heavy lifting  Example  Go through the list from the beginning starting with element 0  find a new random position for it  say 6  put 0   s value in 6 and 6   s value in 0  Move on to element 1 and repeat this process  and so on through the rest of the list  import random iteration   random randint 2  100  temp var   0 while iteration  gt  0       for i in range 1  len my list      have to use range with len           for j in range 1  len my list  - i                 Using temp var as my place holder so I don t lose values             temp var   my list i              my list i    my list j              my list j    temp var          iteration -  1

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def shuffle  list       if not  list                import random         list2              while  list                    card   random choice  list               list remove card              list2 append card          while list2                    card1   list2 0              list2 remove card1               list append card1          return  list

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One can define a function called shuffled  in the same sense of sort vs sorted   def shuffled x       import random     y   x        random shuffle y      return y  x   shuffled  1  2  3  4   print x

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For one-liners  userandom sample list to be shuffled  length of the list  with an example   import random random sample list range 10    10    outputs   2  9  7  8  3  0  4  1  6  5

[python] Shuffling a list of objects

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