Ignore python multiple return value

Question

Say I have a Python function that returns multiple values in a tuple   def func        return 1  2   Is there a nice way to ignore one of the results rather than just assigning to a temporary variable  Say if I was only interested in the first value  is there a better way than this   x  temp   func

User · Accepted Answer

One common convention is to use a     as a variable name for the elements of the tuple you wish to ignore  For instance   def f        return 1  2  3        x   f

User · Answer

You can use x   func   0  to return the first value  x   func   1  to return the second  and so on   If you want to get multiple values at a time  use something like x  y   func   2 4

User · Answer

Remember  when you return more than one item  you re really returning a tuple  So you can do things like this   def func        return 1  2  print func   0    prints 1 print func   1    prints 2

User · Answer

The best solution probably is to name things instead of returning meaningless tuples  unless there is some logic behind the order of the returned items   You can for example use a dictionary  def func        return   lat   1   lng   2       latitude   func    lat    You could even use namedtuple if you want to add extra information about what you are returning  it s not just a dictionary  it s a pair of  coordinates   from collections import namedtuple   Coordinates   namedtuple  Coordinates     lat    lng     def func        return Coordinates lat 1  lng 2   latitude   func   lat  If the objects within your dictionary tuple are strongly tied together then it may be a good idea to even define a class for it  That way you ll also be able to define more complex operations  A natural question that follows is  When should I be using classes in Python  Most recent versions of python    3 7  have dataclasses which you can use to define classes with very few lines of code  from dataclasses import dataclass   dataclass class Coordinates      lat  float   0     lng  float   0  def func        return Coordinates lat 1  lng 2   latitude   func   lat  The primary advantage of dataclasses over namedtuple is that its easier to extend  but there are other differences  Note that by default  dataclasses are mutable  but you can use  dataclass frozen True  instead of  dataclass to force them being immutable

User · Answer

This is not a direct answer to the question  Rather it answers this question   How do I choose a specific function output from many possible options     If you are able to write the function  ie  it is not in a library you cannot modify   then add an input argument that indicates what you want out of the function  Make it a named argument with a default value so in the  common case  you don t even have to specify it       def fancy function  arg1  arg2  return type 1            ret val   None         if  1    return type                ret val   arg1   arg2         elif  2    return type                ret val     arg1  arg2  arg1   arg2           else              ret val     arg1  arg2  arg1   arg2  arg1   arg2            return  ret val     This method gives the function  advanced warning  regarding the desired output  Consequently it can skip unneeded processing and only do the work necessary to get your desired output  Also because Python does dynamic typing  the return type can change  Notice how the example returns a scalar  a list or a tuple    whatever you like

User · Answer

This seems like the best choice to me   val1  val2  ignored1  ignored2   some function     It s not cryptic or ugly  like the func   index  method   and clearly states your purpose

User · Answer

Three simple choices   Obvious  x      func    x  junk   func     Hideous  x   func   0    And there are ways to do this with a decorator   def val0  aFunc        def pick0   args    kw            return aFunc  args   kw  0      return pick0  func0  val0 func

User · Answer

This is not a direct answer to the question  Rather it answers this question   How do I choose a specific function output from many possible options     If you are able to write the function  ie  it is not in a library you cannot modify   then add an input argument that indicates what you want out of the function  Make it a named argument with a default value so in the  common case  you don t even have to specify it       def fancy function  arg1  arg2  return type 1            ret val   None         if  1    return type                ret val   arg1   arg2         elif  2    return type                ret val     arg1  arg2  arg1   arg2           else              ret val     arg1  arg2  arg1   arg2  arg1   arg2            return  ret val     This method gives the function  advanced warning  regarding the desired output  Consequently it can skip unneeded processing and only do the work necessary to get your desired output  Also because Python does dynamic typing  the return type can change  Notice how the example returns a scalar  a list or a tuple    whatever you like

User · Answer

If you re using Python 3  you can you use the star before a variable  on the left side of an assignment  to have it be a list in unpacking     Example 1  a is 1 and b is  2  3   a   b    1  2  3     Example 2  a is 1  b is  2  3   and c is 4  a   b  c    1  2  3  4     Example 3  b is  1  2  and c is 3   b  c    1  2  3            Example 4  a is 1 and b is     a   b    1

User · Answer

This seems like the best choice to me   val1  val2  ignored1  ignored2   some function     It s not cryptic or ugly  like the func   index  method   and clearly states your purpose

User · Answer

If this is a function that you use all the time but always discard the second argument  I would argue that it is less messy to create an alias for the function without the second return value using lambda    def func        return 1  2  func    lambda  func   0    func       Prints 1

User · Answer

The common practice is to use the dummy variable    single underscore   as many have indicated here before   However  to avoid collisions with other uses of that variable name  see this response  it might be a better practice to use     double underscore  instead as a throwaway variable  as pointed by ncoghlan  E g    x       func

User · Answer

This seems like the best choice to me   val1  val2  ignored1  ignored2   some function     It s not cryptic or ugly  like the func   index  method   and clearly states your purpose

User · Answer

When you have many output from a function and you don t want to call it multiple times  I think the clearest way for selecting the results would be    results   fct   a b    results i  for i in list of index    As a minimum working example  also demonstrating that the function is called only once    def fct a       b a 2     c a 2     d a b     e b 2     f a a     print  fct called       return a b c d e f   results fct 3   gt  fct called  x y    results i  for i in  1 4     And the values are as expected    results  gt   3 6 5 9 12 9  x  gt  6 y  gt  12   For convenience  Python list indexes can also be used    x y    results i  for i in  0 -2     Returns   a   3 and b   12

User · Answer

This seems like the best choice to me   val1  val2  ignored1  ignored2   some function     It s not cryptic or ugly  like the func   index  method   and clearly states your purpose

User · Answer

You can use x   func   0  to return the first value  x   func   1  to return the second  and so on   If you want to get multiple values at a time  use something like x  y   func   2 4

User · Answer

If you re using Python 3  you can you use the star before a variable  on the left side of an assignment  to have it be a list in unpacking     Example 1  a is 1 and b is  2  3   a   b    1  2  3     Example 2  a is 1  b is  2  3   and c is 4  a   b  c    1  2  3  4     Example 3  b is  1  2  and c is 3   b  c    1  2  3            Example 4  a is 1 and b is     a   b    1

User · Answer

Three simple choices   Obvious  x      func    x  junk   func     Hideous  x   func   0    And there are ways to do this with a decorator   def val0  aFunc        def pick0   args    kw            return aFunc  args   kw  0      return pick0  func0  val0 func

User · Answer

If you re using Python 3  you can you use the star before a variable  on the left side of an assignment  to have it be a list in unpacking     Example 1  a is 1 and b is  2  3   a   b    1  2  3     Example 2  a is 1  b is  2  3   and c is 4  a   b  c    1  2  3  4     Example 3  b is  1  2  and c is 3   b  c    1  2  3            Example 4  a is 1 and b is     a   b    1

User · Answer

Three simple choices   Obvious  x      func    x  junk   func     Hideous  x   func   0    And there are ways to do this with a decorator   def val0  aFunc        def pick0   args    kw            return aFunc  args   kw  0      return pick0  func0  val0 func

User · Answer

The common practice is to use the dummy variable    single underscore   as many have indicated here before   However  to avoid collisions with other uses of that variable name  see this response  it might be a better practice to use     double underscore  instead as a throwaway variable  as pointed by ncoghlan  E g    x       func

User · Answer

You can use x   func   0  to return the first value  x   func   1  to return the second  and so on   If you want to get multiple values at a time  use something like x  y   func   2 4

User · Answer

The best solution probably is to name things instead of returning meaningless tuples  unless there is some logic behind the order of the returned items   You can for example use a dictionary  def func        return   lat   1   lng   2       latitude   func    lat    You could even use namedtuple if you want to add extra information about what you are returning  it s not just a dictionary  it s a pair of  coordinates   from collections import namedtuple   Coordinates   namedtuple  Coordinates     lat    lng     def func        return Coordinates lat 1  lng 2   latitude   func   lat  If the objects within your dictionary tuple are strongly tied together then it may be a good idea to even define a class for it  That way you ll also be able to define more complex operations  A natural question that follows is  When should I be using classes in Python  Most recent versions of python    3 7  have dataclasses which you can use to define classes with very few lines of code  from dataclasses import dataclass   dataclass class Coordinates      lat  float   0     lng  float   0  def func        return Coordinates lat 1  lng 2   latitude   func   lat  The primary advantage of dataclasses over namedtuple is that its easier to extend  but there are other differences  Note that by default  dataclasses are mutable  but you can use  dataclass frozen True  instead of  dataclass to force them being immutable

User · Answer

If this is a function that you use all the time but always discard the second argument  I would argue that it is less messy to create an alias for the function without the second return value using lambda    def func        return 1  2  func    lambda  func   0    func       Prints 1

User · Answer

If you re using Python 3  you can you use the star before a variable  on the left side of an assignment  to have it be a list in unpacking     Example 1  a is 1 and b is  2  3   a   b    1  2  3     Example 2  a is 1  b is  2  3   and c is 4  a   b  c    1  2  3  4     Example 3  b is  1  2  and c is 3   b  c    1  2  3            Example 4  a is 1 and b is     a   b    1

User · Answer

Remember  when you return more than one item  you re really returning a tuple  So you can do things like this   def func        return 1  2  print func   0    prints 1 print func   1    prints 2

User · Answer

You can use x   func   0  to return the first value  x   func   1  to return the second  and so on   If you want to get multiple values at a time  use something like x  y   func   2 4

User · Answer

When you have many output from a function and you don t want to call it multiple times  I think the clearest way for selecting the results would be    results   fct   a b    results i  for i in list of index    As a minimum working example  also demonstrating that the function is called only once    def fct a       b a 2     c a 2     d a b     e b 2     f a a     print  fct called       return a b c d e f   results fct 3   gt  fct called  x y    results i  for i in  1 4     And the values are as expected    results  gt   3 6 5 9 12 9  x  gt  6 y  gt  12   For convenience  Python list indexes can also be used    x y    results i  for i in  0 -2     Returns   a   3 and b   12

User · Answer

Remember  when you return more than one item  you re really returning a tuple  So you can do things like this   def func        return 1  2  print func   0    prints 1 print func   1    prints 2

User · Answer

Three simple choices   Obvious  x      func    x  junk   func     Hideous  x   func   0    And there are ways to do this with a decorator   def val0  aFunc        def pick0   args    kw            return aFunc  args   kw  0      return pick0  func0  val0 func

User · Answer

Remember  when you return more than one item  you re really returning a tuple  So you can do things like this   def func        return 1  2  print func   0    prints 1 print func   1    prints 2

[python] Ignore python multiple return value

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