What does asterisk mean in Python

Question

Does   have a special meaning in Python as it does in C  I saw a function like this in the Python Cookbook   def get self   a    kw    Would you please explain it to me or point out where I can find an answer  Google interprets the   as wild card character and thus I cannot find a satisfactory answer

User · Answer

All of the above answers were perfectly clear and complete, but just for the record I'd like to confirm that the meaning of * and ** in python has absolutely no similarity with the meaning of similar-looking operators in C.

They are called the argument-unpacking and keyword-argument-unpacking operators.

User · Answer

I find   useful when writing a function that takes another callback function as a parameter   def some function parm1  parm2  callback   callback args       a   1     b   2             callback a  b   callback args            That way  callers can pass in arbitrary extra parameters that will be passed through to their callback function   The nice thing is that the callback function can use normal function parameters   That is  it doesn t need to use the   syntax at all   Here s an example   def my callback function a  b  x  y  z            x   5 y   6 z   7  some function  parm1    parm2   my callback function  x  y  z    Of course  closures provide another way of doing the same thing without requiring you to pass x  y  and z through some function   and into my callback function

User · Answer

A single star means that the variable  a  will be a tuple of extra parameters that were supplied to the function   The double star means the variable  kw  will be a variable-size dictionary of extra parameters that were supplied with keywords   Although the actual behavior is spec d out  it still sometimes can be very non-intuitive   Writing some sample functions and calling them with various parameter styles may help you understand what is allowed and what the results are   def f0 a  def f1  a  def f2   a  def f3  a    b  etc

User · Answer

All of the above answers were perfectly clear and complete, but just for the record I'd like to confirm that the meaning of * and ** in python has absolutely no similarity with the meaning of similar-looking operators in C.

They are called the argument-unpacking and keyword-argument-unpacking operators.

User · Answer

See Function Definitions in the Language Reference      If the form  identifier is   present  it is initialized to a tuple   receiving any excess positional   parameters  defaulting to the empty   tuple  If the form   identifier is   present  it is initialized to a new   dictionary receiving any excess   keyword arguments  defaulting to a new   empty dictionary    Also  see Function Calls   Assuming that one knows what positional and keyword arguments are  here are some examples   Example 1     Excess keyword argument  python 2  example  def foo a  b  c    args       print  a    s     a       print  b    s     b       print  c    s     c       print args  foo a  testa   d  excess   c  testc   b  testb   k  another excess     As you can see in the above example  we only have parameters a  b  c in the signature of the foo function  Since d and k are not present  they are put into the args dictionary  The output of the program is   a   testa b   testb c   testc   k    another excess    d    excess     Example 2     Excess positional argument  python 2  example  def foo a  b  c   args       print  a    s     a       print  b    s     b       print  c    s     c       print args  foo  testa    testb    testc    excess    another excess     Here  since we re testing positional arguments  the excess ones have to be on the end  and  args packs them into a tuple  so the output of this program is   a   testa b   testb c   testc   excess    another excess     You can also unpack a dictionary or a tuple into arguments of a function   def foo a b c   args       print  a  s     a       print  b  s     b       print  c  s     c       print  args  s     args    argdict   dict a  testa   b  testb   c  testc   excessarg  string   foo   argdict    Prints   a testa b testb c testc args   excessarg    string     And  def foo a b c  args       print  a  s     a       print  b  s     b       print  c  s     c       print  args  s     args    argtuple     testa   testb   testc   excess   foo  argtuple    Prints   a testa b testb c testc args   excess

User · Answer

All of the above answers were perfectly clear and complete, but just for the record I'd like to confirm that the meaning of * and ** in python has absolutely no similarity with the meaning of similar-looking operators in C.

They are called the argument-unpacking and keyword-argument-unpacking operators.

User · Answer

I only have one thing to add that wasn t clear from the other answers  for completeness s sake    You may also use the stars when calling the function   For example  say you have code like this    gt  gt  gt  def foo  args           print args       gt  gt  gt  l    1 2 3 4 5    You can pass the list l into foo like so      gt  gt  gt  foo  l   1  2  3  4  5    You can do the same for dictionaries      gt  gt  gt  def foo   argd           print argd       gt  gt  gt  d     a     b    c     d    gt  gt  gt  foo   d    a    b    c    d

User · Answer

I only have one thing to add that wasn t clear from the other answers  for completeness s sake    You may also use the stars when calling the function   For example  say you have code like this    gt  gt  gt  def foo  args           print args       gt  gt  gt  l    1 2 3 4 5    You can pass the list l into foo like so      gt  gt  gt  foo  l   1  2  3  4  5    You can do the same for dictionaries      gt  gt  gt  def foo   argd           print argd       gt  gt  gt  d     a     b    c     d    gt  gt  gt  foo   d    a    b    c    d

User · Answer

I only have one thing to add that wasn t clear from the other answers  for completeness s sake    You may also use the stars when calling the function   For example  say you have code like this    gt  gt  gt  def foo  args           print args       gt  gt  gt  l    1 2 3 4 5    You can pass the list l into foo like so      gt  gt  gt  foo  l   1  2  3  4  5    You can do the same for dictionaries      gt  gt  gt  def foo   argd           print argd       gt  gt  gt  d     a     b    c     d    gt  gt  gt  foo   d    a    b    c    d

User · Answer

See Function Definitions in the Language Reference      If the form  identifier is   present  it is initialized to a tuple   receiving any excess positional   parameters  defaulting to the empty   tuple  If the form   identifier is   present  it is initialized to a new   dictionary receiving any excess   keyword arguments  defaulting to a new   empty dictionary    Also  see Function Calls   Assuming that one knows what positional and keyword arguments are  here are some examples   Example 1     Excess keyword argument  python 2  example  def foo a  b  c    args       print  a    s     a       print  b    s     b       print  c    s     c       print args  foo a  testa   d  excess   c  testc   b  testb   k  another excess     As you can see in the above example  we only have parameters a  b  c in the signature of the foo function  Since d and k are not present  they are put into the args dictionary  The output of the program is   a   testa b   testb c   testc   k    another excess    d    excess     Example 2     Excess positional argument  python 2  example  def foo a  b  c   args       print  a    s     a       print  b    s     b       print  c    s     c       print args  foo  testa    testb    testc    excess    another excess     Here  since we re testing positional arguments  the excess ones have to be on the end  and  args packs them into a tuple  so the output of this program is   a   testa b   testb c   testc   excess    another excess     You can also unpack a dictionary or a tuple into arguments of a function   def foo a b c   args       print  a  s     a       print  b  s     b       print  c  s     c       print  args  s     args    argdict   dict a  testa   b  testb   c  testc   excessarg  string   foo   argdict    Prints   a testa b testb c testc args   excessarg    string     And  def foo a b c  args       print  a  s     a       print  b  s     b       print  c  s     c       print  args  s     args    argtuple     testa   testb   testc   excess   foo  argtuple    Prints   a testa b testb c testc args   excess

User · Answer

I only have one thing to add that wasn t clear from the other answers  for completeness s sake    You may also use the stars when calling the function   For example  say you have code like this    gt  gt  gt  def foo  args           print args       gt  gt  gt  l    1 2 3 4 5    You can pass the list l into foo like so      gt  gt  gt  foo  l   1  2  3  4  5    You can do the same for dictionaries      gt  gt  gt  def foo   argd           print argd       gt  gt  gt  d     a     b    c     d    gt  gt  gt  foo   d    a    b    c    d

User · Answer

See Function Definitions in the Language Reference      If the form  identifier is   present  it is initialized to a tuple   receiving any excess positional   parameters  defaulting to the empty   tuple  If the form   identifier is   present  it is initialized to a new   dictionary receiving any excess   keyword arguments  defaulting to a new   empty dictionary    Also  see Function Calls   Assuming that one knows what positional and keyword arguments are  here are some examples   Example 1     Excess keyword argument  python 2  example  def foo a  b  c    args       print  a    s     a       print  b    s     b       print  c    s     c       print args  foo a  testa   d  excess   c  testc   b  testb   k  another excess     As you can see in the above example  we only have parameters a  b  c in the signature of the foo function  Since d and k are not present  they are put into the args dictionary  The output of the program is   a   testa b   testb c   testc   k    another excess    d    excess     Example 2     Excess positional argument  python 2  example  def foo a  b  c   args       print  a    s     a       print  b    s     b       print  c    s     c       print args  foo  testa    testb    testc    excess    another excess     Here  since we re testing positional arguments  the excess ones have to be on the end  and  args packs them into a tuple  so the output of this program is   a   testa b   testb c   testc   excess    another excess     You can also unpack a dictionary or a tuple into arguments of a function   def foo a b c   args       print  a  s     a       print  b  s     b       print  c  s     c       print  args  s     args    argdict   dict a  testa   b  testb   c  testc   excessarg  string   foo   argdict    Prints   a testa b testb c testc args   excessarg    string     And  def foo a b c  args       print  a  s     a       print  b  s     b       print  c  s     c       print  args  s     args    argtuple     testa   testb   testc   excess   foo  argtuple    Prints   a testa b testb c testc args   excess

User · Answer

A single star means that the variable  a  will be a tuple of extra parameters that were supplied to the function   The double star means the variable  kw  will be a variable-size dictionary of extra parameters that were supplied with keywords   Although the actual behavior is spec d out  it still sometimes can be very non-intuitive   Writing some sample functions and calling them with various parameter styles may help you understand what is allowed and what the results are   def f0 a  def f1  a  def f2   a  def f3  a    b  etc

User · Answer

I find   useful when writing a function that takes another callback function as a parameter   def some function parm1  parm2  callback   callback args       a   1     b   2             callback a  b   callback args            That way  callers can pass in arbitrary extra parameters that will be passed through to their callback function   The nice thing is that the callback function can use normal function parameters   That is  it doesn t need to use the   syntax at all   Here s an example   def my callback function a  b  x  y  z            x   5 y   6 z   7  some function  parm1    parm2   my callback function  x  y  z    Of course  closures provide another way of doing the same thing without requiring you to pass x  y  and z through some function   and into my callback function

User · Answer

All of the above answers were perfectly clear and complete, but just for the record I'd like to confirm that the meaning of * and ** in python has absolutely no similarity with the meaning of similar-looking operators in C.

They are called the argument-unpacking and keyword-argument-unpacking operators.

User · Answer

I find   useful when writing a function that takes another callback function as a parameter   def some function parm1  parm2  callback   callback args       a   1     b   2             callback a  b   callback args            That way  callers can pass in arbitrary extra parameters that will be passed through to their callback function   The nice thing is that the callback function can use normal function parameters   That is  it doesn t need to use the   syntax at all   Here s an example   def my callback function a  b  x  y  z            x   5 y   6 z   7  some function  parm1    parm2   my callback function  x  y  z    Of course  closures provide another way of doing the same thing without requiring you to pass x  y  and z through some function   and into my callback function

User · Answer

A single star means that the variable  a  will be a tuple of extra parameters that were supplied to the function   The double star means the variable  kw  will be a variable-size dictionary of extra parameters that were supplied with keywords   Although the actual behavior is spec d out  it still sometimes can be very non-intuitive   Writing some sample functions and calling them with various parameter styles may help you understand what is allowed and what the results are   def f0 a  def f1  a  def f2   a  def f3  a    b  etc

User · Answer

A single star means that the variable  a  will be a tuple of extra parameters that were supplied to the function   The double star means the variable  kw  will be a variable-size dictionary of extra parameters that were supplied with keywords   Although the actual behavior is spec d out  it still sometimes can be very non-intuitive   Writing some sample functions and calling them with various parameter styles may help you understand what is allowed and what the results are   def f0 a  def f1  a  def f2   a  def f3  a    b  etc

User · Answer

I find   useful when writing a function that takes another callback function as a parameter   def some function parm1  parm2  callback   callback args       a   1     b   2             callback a  b   callback args            That way  callers can pass in arbitrary extra parameters that will be passed through to their callback function   The nice thing is that the callback function can use normal function parameters   That is  it doesn t need to use the   syntax at all   Here s an example   def my callback function a  b  x  y  z            x   5 y   6 z   7  some function  parm1    parm2   my callback function  x  y  z    Of course  closures provide another way of doing the same thing without requiring you to pass x  y  and z through some function   and into my callback function

User · Answer

See Function Definitions in the Language Reference      If the form  identifier is   present  it is initialized to a tuple   receiving any excess positional   parameters  defaulting to the empty   tuple  If the form   identifier is   present  it is initialized to a new   dictionary receiving any excess   keyword arguments  defaulting to a new   empty dictionary    Also  see Function Calls   Assuming that one knows what positional and keyword arguments are  here are some examples   Example 1     Excess keyword argument  python 2  example  def foo a  b  c    args       print  a    s     a       print  b    s     b       print  c    s     c       print args  foo a  testa   d  excess   c  testc   b  testb   k  another excess     As you can see in the above example  we only have parameters a  b  c in the signature of the foo function  Since d and k are not present  they are put into the args dictionary  The output of the program is   a   testa b   testb c   testc   k    another excess    d    excess     Example 2     Excess positional argument  python 2  example  def foo a  b  c   args       print  a    s     a       print  b    s     b       print  c    s     c       print args  foo  testa    testb    testc    excess    another excess     Here  since we re testing positional arguments  the excess ones have to be on the end  and  args packs them into a tuple  so the output of this program is   a   testa b   testb c   testc   excess    another excess     You can also unpack a dictionary or a tuple into arguments of a function   def foo a b c   args       print  a  s     a       print  b  s     b       print  c  s     c       print  args  s     args    argdict   dict a  testa   b  testb   c  testc   excessarg  string   foo   argdict    Prints   a testa b testb c testc args   excessarg    string     And  def foo a b c  args       print  a  s     a       print  b  s     b       print  c  s     c       print  args  s     args    argtuple     testa   testb   testc   excess   foo  argtuple    Prints   a testa b testb c testc args   excess

[python] What does asterisk * mean in Python?

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