What s the difference between lists enclosed by square brackets and parentheses in Python

Question

gt  gt  gt  x  1 2   gt  gt  gt  x 1  2  gt  gt  gt  x  1 2   gt  gt  gt  x 1  2   Are they both valid  Is one preferred for some reason

User · Answer

Another way brackets and parentheses differ is that square brackets can describe a list comprehension, e.g. [x for x in y]

Whereas the corresponding parenthetic syntax specifies a tuple generator: (x for x in y)

You can get a tuple comprehension using: tuple(x for x in y)

See: Why is there no tuple comprehension in Python?

User · Answer

One interesting difference    lst  1  print lst             prints  1  print type lst        prints  lt type  list  gt   notATuple  1  print notATuple           prints 1 print type notATuple      prints  lt type  int  gt                                              instead of tuple expected    A comma must be included in a tuple even if it contains only a single value  e g   1   instead of  1

User · Answer

The first is a list  the second is a tuple  Lists are mutable  tuples are not   Take a look at the Data Structures section of the tutorial  and the Sequence Types section of the documentation

User · Answer

Comma-separated items enclosed by    and   are tuples  those enclosed by   and   are lists

User · Answer

Square brackets are lists while parentheses are tuples   A list is mutable  meaning you can change its contents    gt  gt  gt  x    1 2   gt  gt  gt  x append 3   gt  gt  gt  x  1  2  3    while tuples are not    gt  gt  gt  x    1 2   gt  gt  gt  x  1  2   gt  gt  gt  x append 3  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   tuple  object has no attribute  append    The other main difference is that a tuple is hashable  meaning that you can use it as a key to a dictionary  among other things  For example    gt  gt  gt  x    1 2   gt  gt  gt  y    1 2   gt  gt  gt  z       gt  gt  gt  z x    3  gt  gt  gt  z   1  2   3   gt  gt  gt  z y    4 Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  unhashable type   list      Note that  as many people have pointed out  you can add tuples together  For example    gt  gt  gt  x    1 2   gt  gt  gt  x     3    gt  gt  gt  x  1  2  3    However  this does not mean tuples are mutable  In the example above  a new tuple is constructed by adding together the two tuples as arguments  The original tuple is not modified  To demonstrate this  consider the following    gt  gt  gt  x    1 2   gt  gt  gt  y   x  gt  gt  gt  x     3    gt  gt  gt  x  1  2  3   gt  gt  gt  y  1  2    Whereas  if you were to construct this same example with a list  y would also be updated    gt  gt  gt  x    1  2   gt  gt  gt  y   x  gt  gt  gt  x     3   gt  gt  gt  x  1  2  3   gt  gt  gt  y  1  2  3

User · Answer

They are not lists  they are a list and a tuple  You can read about tuples in the Python tutorial   While you can mutate lists  this is not possible with tuples   In  1   x    1  2   In  2   x 0    3 --------------------------------------------------------------------------- TypeError                                 Traceback  most recent call last    home user  lt ipython console gt  in  lt module gt     TypeError   tuple  object does not support item assignment

[python] What's the difference between lists enclosed by square brackets and parentheses in Python?

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