What does - mean in Python function definitions

Question

I ve recently noticed something interesting when looking at Python 3 3 grammar specification   funcdef   def  NAME parameters   - gt   test      suite   The optional  arrow  block was absent in Python 2 and I couldn t find any information regarding its meaning in Python 3  It turns out this is correct Python and it s accepted by the interpreter   def f x  - gt  123      return x   I thought that this might be some kind of a precondition syntax  but    I cannot test x here  at it is still undefined  No matter what I put after the arrow  e g  2  lt  1   it doesn t affect the function behaviour    Could anyone accustomed with this syntax explain it

User · Answer

def f x  - gt  str  return x 4  print f 45      will give the result    49    or with other words  - gt  str  has NO effect to return type   print f 45    class      lt class  int  gt

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These are function annotations covered in PEP 3107  Specifically  the - gt  marks the return function annotation   Examples    gt  gt  gt  def kinetic energy m  in KG   v  in M S  - gt  Joules           return 1 2 m v  2       gt  gt  gt  kinetic energy   annotations     return    Joules    v    in M S    m    in KG     Annotations are dictionaries  so you can do this    gt  gt  gt            format kinetic energy 20 3000         kinetic energy   annotations    return     90 000 000 0 Joules    You can also have a python data structure rather than just a string    gt  gt  gt  rd   type  float  units   Joules   docstring   Given mass and velocity returns kinetic energy in Joules    gt  gt  gt  def f  - gt rd         pass  gt  gt  gt  f   annotations    return    type    lt class  float  gt   gt  gt  gt  f   annotations    return    units    Joules   gt  gt  gt  f   annotations    return    docstring    Given mass and velocity returns kinetic energy in Joules    Or  you can use function attributes to validate called values   def validate func  locals       for var  test in func   annotations   items            value   locals var          try               pr test   name        test   docstring           except AttributeError              pr test   name              msg            Test      format var  value  pr          assert test value   msg  def between lo  hi       def  between x               return lo  lt   x  lt   hi      between   docstring    must be between    and     format lo hi             return  between  def f x  between 3 10   y lambda  y  isinstance  y int        validate f  locals        print x y    Prints    gt  gt  gt  f 2 2   AssertionError  x  2  Test   between  must be between 3 and 10  gt  gt  gt  f 3 2 1  AssertionError  y  2 1  Test   lt lambda gt

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As other answers have stated  the - gt  symbol is used as part of function annotations  In more recent versions of Python  gt   3 5  though  it has a defined meaning    PEP 3107 -- Function Annotations described the specification  defining the grammar changes  the existence of func   annotations   in which they are stored and  the fact that it s use case is still open    In Python 3 5 though  PEP 484 -- Type Hints attaches a single meaning to this  - gt  is used to indicate the type that the function returns  It also seems like this will be enforced in future versions as described in What about existing uses of annotations      The fastest conceivable scheme would introduce silent deprecation of non-type-hint annotations in 3 6  full deprecation in 3 7  and declare type hints as the only allowed use of annotations in Python 3 8     Emphasis mine   This hasn t been actually implemented as of 3 6 as far as I can tell so it might get bumped to future versions   According to this  the example you ve supplied   def f x  - gt  123      return x   will be forbidden in the future  and in current versions will be confusing   it would need to be changed to   def f x  - gt  int      return x   for it to effectively describe that function f returns an object of type int   The annotations are not used in any way by Python itself  it pretty much populates and ignores them  It s up to 3rd party libraries to work with them

User · Answer

This means the type of result the function returns  but it can be None   It is widespread in modern libraries oriented on Python 3 x   For example  it there is in code of library pandas-profiling in many places for example   def get description self  - gt  dict   def get rejected variables self  threshold  float   0 9  - gt  list   def to file self  output file  Path or str  silent  bool   True  - gt  None     Write the report to a file

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def function arg - gt 123   It s simply a return type  integer in this case doesn t matter which number you write   like Java    public int function int args         But for Python  how Jim Fasarakis Hilliard said    the return type it s just an hint  so it s suggest the return but allow anyway to return other type like a string

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In the following code   def f x  - gt  int      return int x    the - gt  int just tells that f   returns an integer  but it doesn t force the function to return an integer   It is called a return annotation  and can be accessed as f   annotations    return     Python also supports parameter annotations   def f x  float  - gt  int      return int x      float tells people who read the program  and some third-party libraries programs  e  g  pylint  that x should be a float  It is accessed as f   annotations    x    and doesn t have any meaning by itself  See the documentation for more information   https   docs python org 3 reference compound stmts html function-definitions https   www python org dev peps pep-3107

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It s a function annotation   In more detail  Python 2 x has docstrings  which allow you to attach a metadata string to various types of object  This is amazingly handy  so Python 3 extends the feature by allowing you to attach metadata to functions describing their parameters and return values   There s no preconceived use case  but the PEP suggests several  One very handy one is to allow you to annotate parameters with their expected types  it would then be easy to write a decorator that verifies the annotations or coerces the arguments to the right type  Another is to allow parameter-specific documentation instead of encoding it into the docstring

User · Answer

def f x  - gt  123      return x   My summary    Simply - gt  is introduced to get developers to optionally specify the return type of the function  See Python Enhancement Proposal 3107 This is an indication of how things may develop in future as Python is adopted extensively - an indication towards strong typing - this is my personal observation  You can specify types for arguments as well  Specifying return type of the functions and arguments will help in reducing logical errors and improving code enhancements  You can have expressions as return type  for both at function and parameter level  and the result of the expressions can be accessed via annotations object s  return  attribute  annotations will be empty for the expression return value for lambda inline functions

[python] What does -> mean in Python function definitions?

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