Is there a difference between using a dict literal and a dict constructor

Question

Using PyCharm  I noticed it offers to convert a dict literal   d          one    1        two    2         into a dict constructor   d   dict one  1   two  2       Do these different approaches differ in some significant way    While writing this question I noticed that using dict   it seems impossible to specify a numeric key    d    1   one   2   two   is possible  but  obviously  dict 1  one       is not  Anything else

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From python 2 7 tutorial      A pair of braces creates an empty   dictionary      Placing a   comma-separated list of key value   pairs within the braces adds initial   key value pairs to the dictionary    this is also the way dictionaries are   written on output    tel     jack   4098   sape   4139  data    k v for k v in zip xrange 10   xrange 10 20      While      The dict   constructor builds   dictionaries directly from lists of   key-value pairs stored as tuples  When   the pairs form a pattern  list   comprehensions can compactly specify   the key-value list    tel   dict    sape   4139     guido   4127     jack   4098      sape   4139   jack   4098   guido   4127  data   dict  k v  for k v in zip xrange 10   xrange 10 20         When the keys are simple strings  it   is sometimes easier to specify pairs   using keyword arguments    dict sape 4139  guido 4127  jack 4098   gt  gt  gt     sape   4139   jack  4098   guido   4127    So both    and dict   produce dictionary but provide a bit different ways of dictionary data initialization

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These two approaches produce identical dictionaries  except  as you ve noted  where the lexical rules of Python interfere   Dictionary literals are a little more obviously dictionaries  and you can create any kind of key  but you need to quote the key names   On the other hand  you can use variables for keys if you need to for some reason   a    hello  d         a   hi          The dict   constructor gives you more flexibility because of the variety of forms of input it takes   For example  you can provide it with an iterator of pairs  and it will treat them as key value pairs   I have no idea why PyCharm would offer to convert one form to the other

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One big difference with python 3 4   pycharm is that the dict   constructor produces a  syntax error  message if the number of keys exceeds 256   I prefer using the dict literal now

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I think you have pointed out the most obvious difference  Apart from that    the first doesn t need to lookup dict which should make it a tiny bit faster   the second looks up dict in locals   and then globals   and the finds the builtin  so you can switch the behaviour by defining a local called dict for example although I can t think of anywhere this would be a good idea apart from maybe when debugging

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the dict   literal is nice when you are copy pasting values from something else  none python  For example a list of environment variables  if you had a bash file  say   FOO  bar  CABBAGE  good    you can easily paste then into a dict   literal and add comments  It also makes it easier to do the opposite  copy into something else  Whereas the   FOO    bar   syntax is pretty unique to python and json  So if you use json a lot  you might want to use    literals with double quotes

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There is no dict literal to create dict-inherited classes  custom dict classes with additional methods  In such case custom dict class constructor should be used  for example   class NestedDict dict              skipped  state type map   NestedDict          owns    Another        uses    Another

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I find the dict literal d     one    1   to be much more readable  your defining data  rather than assigning things values and sending them to the dict   constructor   On the other hand i have seen people mistype the dict literal as d     one    1   which in modern python 2 7  will create a set   Despite this i still prefer to all-ways use the set literal because i think its more readable  personal preference i suppose

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Literal is much faster  since it uses optimized BUILD MAP and STORE MAP opcodes rather than generic CALL FUNCTION    gt  python2 7 -m timeit  d   dict a 1  b 2  c 3  d 4  e 5   1000000 loops  best of 3  0 958 usec per loop   gt  python2 7 -m timeit  d     a  1   b  2   c  3   d  4   e  5   1000000 loops  best of 3  0 479 usec per loop   gt  python3 2 -m timeit  d   dict a 1  b 2  c 3  d 4  e 5   1000000 loops  best of 3  0 975 usec per loop   gt  python3 2 -m timeit  d     a  1   b  2   c  3   d  4   e  5   1000000 loops  best of 3  0 409 usec per loop

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They look pretty much the same on Python 3 2   As gnibbler pointed out  the first doesn t need to lookup dict  which should make it a tiny bit faster    gt  gt  gt  def literal          d     one   1   two   2       gt  gt  gt  def constructor          d   dict one  1   two  2        gt  gt  gt  import dis  gt  gt  gt  dis dis literal    2           0 BUILD MAP                2               3 LOAD CONST               1  1                6 LOAD CONST               2   one                 9 STORE MAP              10 LOAD CONST               3  2               13 LOAD CONST               4   two                16 STORE MAP              17 STORE FAST               0  d               20 LOAD CONST               0  None               23 RETURN VALUE  gt  gt  gt  dis dis constructor    2           0 LOAD GLOBAL              0  dict                3 LOAD CONST               1   one                 6 LOAD CONST               2   1                 9 LOAD CONST               3   two                12 LOAD CONST               4   2                15 CALL FUNCTION          512              18 STORE FAST               0  d               21 LOAD CONST               0  None               24 RETURN VALUE

User · Answer

Also consider the fact that tokens that match for operators can t be used in the constructor syntax  i e  dasherized keys    gt  gt  gt  dict foo-bar 1  File   lt stdin gt    line 1 SyntaxError  keyword can t be an expression   gt  gt  gt    foo-bar   1    foo-bar   1

[python] Is there a difference between using a dict literal and a dict constructor?

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