Create list of single item repeated N times

Question

I want to create a series of lists  all of varying lengths  Each list will contain the same element e  repeated n times  where n   length of the list    How do I create the lists  without using a list comprehension  e for number in xrange n   for each list

User · Answer

As others have pointed out  using the   operator for a mutable object duplicates references  so if you change one you change them all  If you want to create independent instances of a mutable object  your xrange syntax is the most Pythonic way to do this  If you are bothered by having a named variable that is never used  you can use the anonymous underscore variable    e for   in xrange n

User · Answer

gt  gt  gt   5    4  5  5  5  5    Be careful when the item being repeated is a list  The list will not be cloned  all the elements will refer to the same list    gt  gt  gt  x  5   gt  gt  gt  y  x    4  gt  gt  gt  y   5    5    5    5    gt  gt  gt  y 0  0    6  gt  gt  gt  y   6    6    6    6

User · Answer

e    n   should work

User · Answer

Itertools has a function just for that   import itertools it   itertools repeat e n    Of course itertools gives you a iterator instead of a list   e    n gives you a list  but  depending on what you will do with those sequences  the itertools variant can be much more efficient

User · Answer

You can also write    e    n   You should note that if e is for example an empty list you get a list with n references to the same list  not n independent empty lists    Performance testing  At first glance it seems that repeat is the fastest way to create a list with n identical elements    gt  gt  gt  timeit timeit  itertools repeat 0  10     import itertools   number   1000000  0 37095273281943264  gt  gt  gt  timeit timeit   0    10    import itertools   number   1000000  0 5577236771712819   But wait - it s not a fair test      gt  gt  gt  itertools repeat 0  10  repeat 0  10     Not a list      The function itertools repeat doesn t actually create the list  it just creates an object that can be used to create a list if you wish  Let s try that again  but converting to a list    gt  gt  gt  timeit timeit  list itertools repeat 0  10      import itertools   number   1000000  1 7508119747063233   So if you want a list  use  e    n  If you want to generate the elements lazily  use repeat

User · Answer

Create List of Single Item Repeated n Times in Python  Depending on your use-case  you want to use different techniques with different semantics  Multiply a list for Immutable items For immutable items  like None  bools  ints  floats  strings  tuples  or frozensets  you can do it like this   e    4  Note that this is usually only used with immutable items  strings  tuples  frozensets    in the list  because they all point to the same item in the same place in memory  I use this frequently when I have to build a table with a schema of all strings  so that I don t have to give a highly redundant one to one mapping  schema     string     len columns   Multiply the list where we want the same item repeated Multiplying a list gives us the same elements over and over  The need for this is rare   iter iterable     4  This is sometimes used to map an iterable into a list of lists   gt  gt  gt  iterable   range 12   gt  gt  gt  a list    iter iterable     4  gt  gt  gt    next l  for l in a list  for i in range 3     0  1  2  3    4  5  6  7    8  9  10  11    We can see that a list contains the same range iterator four times   gt  gt  gt  a list   lt range iterator object at 0x7fde73a5da20 gt    lt range iterator object at 0x7fde73a5da20 gt    lt range iterator object at 0x7fde73a5da20 gt    lt range iterator object at 0x7fde73a5da20 gt    Mutable items I ve used Python for a long time now  and I have seen very few use-cases where I would do the above with mutable objects  Instead  to get  say  a mutable empty list  set  or dict  you should do something like this  list of lists       for   in columns   The underscore is simply a throwaway variable name in this context  If you only have the number  that would be  list of lists       for   in range 4    The   is not really special  but your coding environment style checker will probably complain if you don t intend to use the variable and use any other name   Caveats for using the immutable method with mutable items  Beware doing this with mutable objects  when you change one of them  they all change because they re all the same object  foo          4 foo 0  append  x    foo now returns     x      x      x      x     But with immutable objects  you can make it work because you change the reference  not the object   gt  gt  gt  l    0    4  gt  gt  gt  l 0     1  gt  gt  gt  l  1  0  0  0    gt  gt  gt  l    frozenset      4  gt  gt  gt  l 0     set  abc    gt  gt  gt  l  frozenset   a    c    b     frozenset      frozenset      frozenset       But again  mutable objects are no good for this  because in-place operations change the object  not the reference  l    set      4  gt  gt  gt  l 0     set  abc        gt  gt  gt  l  set   a    c    b     set   a    c    b     set   a    c    b     set   a    c    b

[python] Create list of single item repeated N times

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