Python print a generator expression

Question

In the Python shell  if I enter a list comprehension such as    gt  gt  gt   x for x in string letters if x in  y for y in  BigMan on campus      I get a nicely printed result     a    c    g    i    m    n    o    p    s    u    B    M     Same for a dictionary comprehension    gt  gt  gt   x x 2 for x in range 1 10    1  2  2  4  3  6  4  8  5  10  6  12  7  14  8  16  9  18    If I enter a generator expression  I get not such a friendly response     gt  gt  gt   x for x in string letters if x in  y for y in  BigMan on campus     lt generator object  lt genexpr gt  at 0x1004a0be0 gt    I know I can do this    gt  gt  gt  for i in    print i  a c g i m n o p s u B M   Other than that  or writing a helper function  can I easily evaluate and print that generator object in the interactive shell

User · Answer

gt  gt  gt  list x for x in string letters if x in  y for y in  BigMan on campus      a    c    g    i    m    n    o    p    s    u    B    M

User · Answer

Unlike a list or a dictionary  a generator can be infinite  Doing this wouldn t work   def gen        x   0     while True          yield x         x    1 g1   gen   list g1      never ends   Also  reading a generator changes it  so there s not a perfect way to view it  To see a sample of the generator s output  you could do  g1   gen    g1 next   for i in range 10

User · Answer

Or you can always map over an iterator  without the need to build an intermediate list    gt  gt  gt      map sys stdout write   x for x in string letters if x in  y for y in  BigMan on campus     acgimnopsuBM

User · Answer

You can just wrap the expression in a call to list    gt  gt  gt  list x for x in string letters if x in  y for y in  BigMan on campus      a    c    g    i    m    n    o    p    s    u    B    M

User · Answer

Quick answer   Doing list   around a generator expression is  almost  exactly equivalent to having     brackets around it  So yeah  you can do   gt  gt  gt  list  x for x in string letters if x in  y for y in  BigMan on campus       But you can just as well do   gt  gt  gt   x for x in string letters if x in  y for y in  BigMan on campus      Yes  that will turn the generator expression into a list comprehension  It s the same thing and calling list   on it  So the way to make a generator expression into a list is to put brackets around it   Detailed explanation   A generator expression is a  naked  for expression  Like so   x x for x in range 10    Now  you can t stick that on a line by itself  you ll get a syntax error  But you can put parenthesis around it    gt  gt  gt   x x for x in range 10    lt generator object  lt genexpr gt  at 0xb7485464 gt    This is sometimes called a generator comprehension  although I think the official name still is generator expression  there isn t really any difference  the parenthesis are only there to make the syntax valid  You do not need them if you are passing it in as the only parameter to a function for example    gt  gt  gt  sorted x x for x in range 10    0  1  4  9  16  25  36  49  64  81    Basically all the other comprehensions available in Python 3 and Python 2 7 is just syntactic sugar around a generator expression  Set comprehensions    gt  gt  gt   x x for x in range 10    0  1  4  81  64  9  16  49  25  36    gt  gt  gt  set x x for x in range 10    0  1  4  81  64  9  16  49  25  36    Dict comprehensions    gt  gt  gt  dict  x  x x  for x in range 10    0  0  1  1  2  4  3  9  4  16  5  25  6  36  7  49  8  64  9  81    gt  gt  gt   x  x x for x in range 10    0  0  1  1  2  4  3  9  4  16  5  25  6  36  7  49  8  64  9  81    And list comprehensions under Python 3    gt  gt  gt  list x x for x in range 10    0  1  4  9  16  25  36  49  64  81    gt  gt  gt   x x for x in range 10    0  1  4  9  16  25  36  49  64  81    Under Python 2  list comprehensions is not just syntactic sugar  But the only difference is that x will under Python 2 leak into the namespace    gt  gt  gt  x 9   While under Python 3 you ll get    gt  gt  gt  x Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  NameError  name  x  is not defined   This means that the best way to get a nice printout of the content of your generator expression in Python is to make a list comprehension out of it  However  this will obviously not work if you already have a generator object  Doing that will just make a list of one generator    gt  gt  gt  foo    x x for x in range 10    gt  gt  gt   foo    lt generator object  lt genexpr gt  at 0xb7559504 gt     In that case you will need to call list      gt  gt  gt  list foo   0  1  4  9  16  25  36  49  64  81    Although this works  but is kinda stupid    gt  gt  gt   x for x in foo   0  1  4  9  16  25  36  49  64  81

[python] Python: print a generator expression?

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