Python object repr self should be an expression

Question

I was looking at the builtin object methods in the Python documentation  and I was interested in the documentation for object   repr   self   Here s what it says      Called by the repr   built-in function   and by string conversions  reverse   quotes  to compute the    official      string representation of an object  If   at all possible  this should look like   a valid Python expression that could   be used to recreate an object with the   same value  given an appropriate   environment   If this is not possible    a string of the form  lt    some useful   description     should be returned    The return value must be a string   object  If a class defines repr     but not str    then repr   is   also used when an    informal    string   representation of instances of that   class is required       This is typically used for debugging    so it is important that the   representation is information-rich and   unambiguous   The most interesting part to me  was        If at all possible  this should look like a valid Python expression that could be used to recreate an object with the same value       but I m not sure exactly what this means  It says it should look like an expression which can be used to recreate the object  but does that mean it should just be an example of the sort of expression you could use  or should it be an actual expression  that can be executed  eval etc    to recreate the object  Or    should it be just a rehasing of the actual expression which was used  for pure information purposes   In general I m a bit confused as to exactly what I should be putting here

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Guideline  If you can succinctly provide an exact representation  format it as a Python expression  which implies that it can be both eval d and copied directly into source code  in the right context   If providing an inexact representation  use  lt     gt  format   There are many possible representations for any value  but the one that s most interesting for Python programmers is an expression that recreates the value   Remember that those who understand Python are the target audience mdash and that s also why inexact representations should include relevant context   Even the default  lt XXX object at 0xNNN gt   while almost entirely useless  still provides type  id    to distinguish different objects   and indication that no better representation is available

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repr  means represention  First we create an instance of class coordinate   x   Coordinate 3  4     Then if we input x into console th output is    lt   main   Coordinate at 0x7fcd40ab27b8 gt    If you use repr      gt  gt  gt  repr x  Coordinate 3  4    the output is as same as  Coordinate 3  4   except it is a string You can use it to recreate a instance of coordinate   In conclusion repr   meathod is print out a string which is the representation of the obeject

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I think the confusion over here roots from the english  I mean   repr      short for  representation  of the value I m guessing  like  S Lott said       What is the difference between  an actual expression  that can     recreate the object  and  a rehasing of the actual expression which was used  to create the object    Both are an expression that creates the object  There s no practical distinction between these  A repr call could produce either a new expression or the original expression  In many cases  they re the same     But in some cases they might be different  E g  coordinate points  you might want c coordinate to return  3 5 but c   repr   to return Coordinate 3  5   Hope that makes more sense

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It should be a Python expression that  when eval d  creates an object with the exact same properties as this one  For example  if you have a Fraction class that contains two integers  a numerator and denominator  your   repr     method would look like this     in the definition of Fraction class def   repr   self       return  Fraction  d   d      self numerator  self denominator    Assuming that the constructor takes those two values

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It should be a Python expression that  when eval d  creates an object with the exact same properties as this one  For example  if you have a Fraction class that contains two integers  a numerator and denominator  your   repr     method would look like this     in the definition of Fraction class def   repr   self       return  Fraction  d   d      self numerator  self denominator    Assuming that the constructor takes those two values

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repr  means represention  First we create an instance of class coordinate   x   Coordinate 3  4     Then if we input x into console th output is    lt   main   Coordinate at 0x7fcd40ab27b8 gt    If you use repr      gt  gt  gt  repr x  Coordinate 3  4    the output is as same as  Coordinate 3  4   except it is a string You can use it to recreate a instance of coordinate   In conclusion repr   meathod is print out a string which is the representation of the obeject

User · Answer

but does that mean it should just be an example of the sort of expression you could use  or should it be an actual expression  that can be executed  eval etc    to recreate the object  Or    should it be just a rehasing of the actual expression which was used  for pure information purposes    Wow  that s a lot of hand-wringing    An  an example of the sort of expression you could use  would not be a representation of a specific object   That can t be useful or meaningful    What is the difference between  an actual expression  that can     recreate the object  and  a rehasing of the actual expression which was used  to create the object    Both are an expression that creates the object   There s no practical distinction between these   A repr call could produce either a new expression or the original expression   In many cases  they re the same    Note that this isn t always possible  practical or desirable   In some cases  you ll notice that repr   presents a string which is clearly not an expression of any kind   The default repr   for any class you define isn t useful as an expression   In some cases  you might have mutual  or circular  references between objects   The repr   of that tangled hierarchy can t make sense   In many cases  an object is built incrementally via a parser   For example  from XML or JSON or something   What would the repr be   The original XML or JSON   Clearly not  since they re not Python   It could be some Python expression that generated the XML   However  for a gigantic XML document  it might not be possible to write a single Python expression that was the functional equivalent of parsing XML

User · Answer

Guideline  If you can succinctly provide an exact representation  format it as a Python expression  which implies that it can be both eval d and copied directly into source code  in the right context   If providing an inexact representation  use  lt     gt  format   There are many possible representations for any value  but the one that s most interesting for Python programmers is an expression that recreates the value   Remember that those who understand Python are the target audience mdash and that s also why inexact representations should include relevant context   Even the default  lt XXX object at 0xNNN gt   while almost entirely useless  still provides type  id    to distinguish different objects   and indication that no better representation is available

User · Answer

Guideline  If you can succinctly provide an exact representation  format it as a Python expression  which implies that it can be both eval d and copied directly into source code  in the right context   If providing an inexact representation  use  lt     gt  format   There are many possible representations for any value  but the one that s most interesting for Python programmers is an expression that recreates the value   Remember that those who understand Python are the target audience mdash and that s also why inexact representations should include relevant context   Even the default  lt XXX object at 0xNNN gt   while almost entirely useless  still provides type  id    to distinguish different objects   and indication that no better representation is available

User · Answer

It should be a Python expression that  when eval d  creates an object with the exact same properties as this one  For example  if you have a Fraction class that contains two integers  a numerator and denominator  your   repr     method would look like this     in the definition of Fraction class def   repr   self       return  Fraction  d   d      self numerator  self denominator    Assuming that the constructor takes those two values

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gt  gt  gt  from datetime import date  gt  gt  gt   gt  gt  gt  repr date today             calls date today     repr      datetime date 2009  1  16    gt  gt  gt  eval                          is the output of the last command datetime date 2009  1  16    The output is a string that can be parsed by the python interpreter and results in an equal object   If that s not possible  it should return a string in the form of  lt    some useful description    gt

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gt  gt  gt  from datetime import date  gt  gt  gt   gt  gt  gt  repr date today             calls date today     repr      datetime date 2009  1  16    gt  gt  gt  eval                          is the output of the last command datetime date 2009  1  16    The output is a string that can be parsed by the python interpreter and results in an equal object   If that s not possible  it should return a string in the form of  lt    some useful description    gt

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To see how the repr works within a class  run the following code  first with and then without the repr method     class Coordinate  object       def   init   self x y           self x   x         self y   y      def getX self             Getter method for a Coordinate object s x coordinate            Getter methods are better practice than just accessing an attribute directly         return self x     def getY self              Getter method for a Coordinate object s y coordinate         return self y      def   repr   self     remove this and the next line and re-run         return  Coordinate     str self getX            str self getY             gt  gt  gt c   Coordinate 2 -8   gt  gt  gt print c

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To see how the repr works within a class  run the following code  first with and then without the repr method     class Coordinate  object       def   init   self x y           self x   x         self y   y      def getX self             Getter method for a Coordinate object s x coordinate            Getter methods are better practice than just accessing an attribute directly         return self x     def getY self              Getter method for a Coordinate object s y coordinate         return self y      def   repr   self     remove this and the next line and re-run         return  Coordinate     str self getX            str self getY             gt  gt  gt c   Coordinate 2 -8   gt  gt  gt print c

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but does that mean it should just be an example of the sort of expression you could use  or should it be an actual expression  that can be executed  eval etc    to recreate the object  Or    should it be just a rehasing of the actual expression which was used  for pure information purposes    Wow  that s a lot of hand-wringing    An  an example of the sort of expression you could use  would not be a representation of a specific object   That can t be useful or meaningful    What is the difference between  an actual expression  that can     recreate the object  and  a rehasing of the actual expression which was used  to create the object    Both are an expression that creates the object   There s no practical distinction between these   A repr call could produce either a new expression or the original expression   In many cases  they re the same    Note that this isn t always possible  practical or desirable   In some cases  you ll notice that repr   presents a string which is clearly not an expression of any kind   The default repr   for any class you define isn t useful as an expression   In some cases  you might have mutual  or circular  references between objects   The repr   of that tangled hierarchy can t make sense   In many cases  an object is built incrementally via a parser   For example  from XML or JSON or something   What would the repr be   The original XML or JSON   Clearly not  since they re not Python   It could be some Python expression that generated the XML   However  for a gigantic XML document  it might not be possible to write a single Python expression that was the functional equivalent of parsing XML

User · Answer

but does that mean it should just be an example of the sort of expression you could use  or should it be an actual expression  that can be executed  eval etc    to recreate the object  Or    should it be just a rehasing of the actual expression which was used  for pure information purposes    Wow  that s a lot of hand-wringing    An  an example of the sort of expression you could use  would not be a representation of a specific object   That can t be useful or meaningful    What is the difference between  an actual expression  that can     recreate the object  and  a rehasing of the actual expression which was used  to create the object    Both are an expression that creates the object   There s no practical distinction between these   A repr call could produce either a new expression or the original expression   In many cases  they re the same    Note that this isn t always possible  practical or desirable   In some cases  you ll notice that repr   presents a string which is clearly not an expression of any kind   The default repr   for any class you define isn t useful as an expression   In some cases  you might have mutual  or circular  references between objects   The repr   of that tangled hierarchy can t make sense   In many cases  an object is built incrementally via a parser   For example  from XML or JSON or something   What would the repr be   The original XML or JSON   Clearly not  since they re not Python   It could be some Python expression that generated the XML   However  for a gigantic XML document  it might not be possible to write a single Python expression that was the functional equivalent of parsing XML

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gt  gt  gt  from datetime import date  gt  gt  gt   gt  gt  gt  repr date today             calls date today     repr      datetime date 2009  1  16    gt  gt  gt  eval                          is the output of the last command datetime date 2009  1  16    The output is a string that can be parsed by the python interpreter and results in an equal object   If that s not possible  it should return a string in the form of  lt    some useful description    gt

User · Answer

It should be a Python expression that  when eval d  creates an object with the exact same properties as this one  For example  if you have a Fraction class that contains two integers  a numerator and denominator  your   repr     method would look like this     in the definition of Fraction class def   repr   self       return  Fraction  d   d      self numerator  self denominator    Assuming that the constructor takes those two values

User · Answer

I think the confusion over here roots from the english  I mean   repr      short for  representation  of the value I m guessing  like  S Lott said       What is the difference between  an actual expression  that can     recreate the object  and  a rehasing of the actual expression which was used  to create the object    Both are an expression that creates the object  There s no practical distinction between these  A repr call could produce either a new expression or the original expression  In many cases  they re the same     But in some cases they might be different  E g  coordinate points  you might want c coordinate to return  3 5 but c   repr   to return Coordinate 3  5   Hope that makes more sense

User · Answer

gt  gt  gt  from datetime import date  gt  gt  gt   gt  gt  gt  repr date today             calls date today     repr      datetime date 2009  1  16    gt  gt  gt  eval                          is the output of the last command datetime date 2009  1  16    The output is a string that can be parsed by the python interpreter and results in an equal object   If that s not possible  it should return a string in the form of  lt    some useful description    gt

User · Answer

Guideline  If you can succinctly provide an exact representation  format it as a Python expression  which implies that it can be both eval d and copied directly into source code  in the right context   If providing an inexact representation  use  lt     gt  format   There are many possible representations for any value  but the one that s most interesting for Python programmers is an expression that recreates the value   Remember that those who understand Python are the target audience mdash and that s also why inexact representations should include relevant context   Even the default  lt XXX object at 0xNNN gt   while almost entirely useless  still provides type  id    to distinguish different objects   and indication that no better representation is available

User · Answer

but does that mean it should just be an example of the sort of expression you could use  or should it be an actual expression  that can be executed  eval etc    to recreate the object  Or    should it be just a rehasing of the actual expression which was used  for pure information purposes    Wow  that s a lot of hand-wringing    An  an example of the sort of expression you could use  would not be a representation of a specific object   That can t be useful or meaningful    What is the difference between  an actual expression  that can     recreate the object  and  a rehasing of the actual expression which was used  to create the object    Both are an expression that creates the object   There s no practical distinction between these   A repr call could produce either a new expression or the original expression   In many cases  they re the same    Note that this isn t always possible  practical or desirable   In some cases  you ll notice that repr   presents a string which is clearly not an expression of any kind   The default repr   for any class you define isn t useful as an expression   In some cases  you might have mutual  or circular  references between objects   The repr   of that tangled hierarchy can t make sense   In many cases  an object is built incrementally via a parser   For example  from XML or JSON or something   What would the repr be   The original XML or JSON   Clearly not  since they re not Python   It could be some Python expression that generated the XML   However  for a gigantic XML document  it might not be possible to write a single Python expression that was the functional equivalent of parsing XML

[python] Python object.repr(self) should be an expression?

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[python] Python object.repr(self) should be an expression?