Why does Python code use len function instead of a length method

Question

I know that python has a len   function that is used to determine the size of a string  but I was wondering why it s not a method of the string object   Update  Ok  I realized I was embarrassingly mistaken    len     is actually a method of a string object  It just seems weird to see object oriented code in Python using the len function on string objects  Furthermore  it s also weird to see   len   as the name instead of just len

User · Accepted Answer

Strings do have a length method    len      The protocol in Python is to implement this method on objects which have a length and use the built-in len   function  which calls it for you  similar to the way you would implement   iter     and use the built-in iter   function  or have the method called behind the scenes for you  on objects which are iterable   See Emulating container types for more information   Here s a good read on the subject of protocols in Python  Python and the Principle of Least Astonishment

User · Answer

There is a len method    gt  gt  gt  a    a string of some length   gt  gt  gt  a   len     23  gt  gt  gt  a   len    lt method-wrapper    len    of str object at 0x02005650 gt

User · Answer

There is a len method    gt  gt  gt  a    a string of some length   gt  gt  gt  a   len     23  gt  gt  gt  a   len    lt method-wrapper    len    of str object at 0x02005650 gt

User · Answer

Jim s answer to this question may help  I copy it here  Quoting Guido van Rossum   First of all  I chose len x  over x len   for HCI reasons  def   len     came much later   There are two intertwined reasons actually  both HCI   a  For some operations  prefix notation just reads better than postfix     prefix  and infix   operations have a long tradition in mathematics which likes notations where the visuals help the mathematician thinking about a problem  Compare the easy with which we rewrite a formula like x  a b  into xa   xb to the clumsiness of doing the same thing using a raw OO notation   b  When I read code that says len x  I know that it is asking for the length of something  This tells me two things  the result is an integer  and the argument is some kind of container  To the contrary  when I read x len    I have to already know that x is some kind of container implementing an interface or inheriting from a class that has a standard len    Witness the confusion we occasionally have when a class that is not implementing a mapping has a get   or keys   method  or something that isn   t a file has a write   method  Saying the same thing in another way  I see    len    as a built-in operation  I   d hate to lose that

User · Answer

Jim s answer to this question may help  I copy it here  Quoting Guido van Rossum   First of all  I chose len x  over x len   for HCI reasons  def   len     came much later   There are two intertwined reasons actually  both HCI   a  For some operations  prefix notation just reads better than postfix     prefix  and infix   operations have a long tradition in mathematics which likes notations where the visuals help the mathematician thinking about a problem  Compare the easy with which we rewrite a formula like x  a b  into xa   xb to the clumsiness of doing the same thing using a raw OO notation   b  When I read code that says len x  I know that it is asking for the length of something  This tells me two things  the result is an integer  and the argument is some kind of container  To the contrary  when I read x len    I have to already know that x is some kind of container implementing an interface or inheriting from a class that has a standard len    Witness the confusion we occasionally have when a class that is not implementing a mapping has a get   or keys   method  or something that isn   t a file has a write   method  Saying the same thing in another way  I see    len    as a built-in operation  I   d hate to lose that

User · Answer

It doesn t    gt  gt  gt   abc    len     3

User · Answer

There is a len method    gt  gt  gt  a    a string of some length   gt  gt  gt  a   len     23  gt  gt  gt  a   len    lt method-wrapper    len    of str object at 0x02005650 gt

User · Answer

Something missing from the rest of the answers here  the len function checks that the   len   method returns a non-negative int  The fact that len is a function means that classes cannot override this behaviour to avoid the check  As such  len obj  gives a level of safety that obj len   cannot   Example    gt  gt  gt  class A          def   len   self               return  foo       gt  gt  gt  len A    Traceback  most recent call last     File   lt pyshell 8 gt    line 1  in  lt module gt      len A    TypeError   str  object cannot be interpreted as an integer  gt  gt  gt  class B          def   len   self               return -1       gt  gt  gt  len B    Traceback  most recent call last     File   lt pyshell 13 gt    line 1  in  lt module gt      len B    ValueError    len     should return  gt   0   Of course  it is possible to  override  the len function by reassigning it as a global variable  but code which does this is much more obviously suspicious than code which overrides a method in a class

User · Answer

met  python -c  import this    grep  only one  There should be one-- and preferably only one --obvious way to do it

User · Answer

Something missing from the rest of the answers here  the len function checks that the   len   method returns a non-negative int  The fact that len is a function means that classes cannot override this behaviour to avoid the check  As such  len obj  gives a level of safety that obj len   cannot   Example    gt  gt  gt  class A          def   len   self               return  foo       gt  gt  gt  len A    Traceback  most recent call last     File   lt pyshell 8 gt    line 1  in  lt module gt      len A    TypeError   str  object cannot be interpreted as an integer  gt  gt  gt  class B          def   len   self               return -1       gt  gt  gt  len B    Traceback  most recent call last     File   lt pyshell 13 gt    line 1  in  lt module gt      len B    ValueError    len     should return  gt   0   Of course  it is possible to  override  the len function by reassigning it as a global variable  but code which does this is much more obviously suspicious than code which overrides a method in a class

User · Answer

You can also say    gt  gt  x    test   gt  gt  len x  4   Using Python 2 7 3

User · Answer

It doesn t    gt  gt  gt   abc    len     3

User · Answer

It doesn t    gt  gt  gt   abc    len     3

User · Answer

There are some great answers here  and so before I give my own I d like to highlight a few of the gems  no ruby pun intended  I ve read here    Python is not a pure OOP language -- it s a general purpose  multi-paradigm language that allows the programmer to use the paradigm they are most comfortable with and or the paradigm that is best suited for their solution  Python has first-class functions  so len is actually an object  Ruby  on the other hand  doesn t have first class functions  So the len function object has it s own methods that you can inspect by running dir len     If you don t like the way this works in your own code  it s trivial for you to re-implement the containers using your preferred method  see example below     gt  gt  gt  class List list           def len self               return len self       gt  gt  gt  class Dict dict           def len self               return len self       gt  gt  gt  class Tuple tuple           def len self               return len self       gt  gt  gt  class Set set           def len self               return len self       gt  gt  gt  my list   List  1 2 3 4 5 6 7 8 9  A   B   C   D   E   F     gt  gt  gt  my dict   Dict   key    value    site    stackoverflow     gt  gt  gt  my set   Set  1 2 3 4 5 6 7 8 9  A   B   C   D   E   F     gt  gt  gt  my tuple   Tuple  1 2 3 4 5 6 7 8 9  A   B   C   D   E   F     gt  gt  gt  my containers   Tuple  my list  my dict  my set  my tuple    gt  gt  gt   gt  gt  gt  for container in my containers          print container len       15 2 15 15

User · Answer

Jim s answer to this question may help  I copy it here  Quoting Guido van Rossum   First of all  I chose len x  over x len   for HCI reasons  def   len     came much later   There are two intertwined reasons actually  both HCI   a  For some operations  prefix notation just reads better than postfix     prefix  and infix   operations have a long tradition in mathematics which likes notations where the visuals help the mathematician thinking about a problem  Compare the easy with which we rewrite a formula like x  a b  into xa   xb to the clumsiness of doing the same thing using a raw OO notation   b  When I read code that says len x  I know that it is asking for the length of something  This tells me two things  the result is an integer  and the argument is some kind of container  To the contrary  when I read x len    I have to already know that x is some kind of container implementing an interface or inheriting from a class that has a standard len    Witness the confusion we occasionally have when a class that is not implementing a mapping has a get   or keys   method  or something that isn   t a file has a write   method  Saying the same thing in another way  I see    len    as a built-in operation  I   d hate to lose that

User · Answer

It doesn t    gt  gt  gt   abc    len     3

User · Answer

met  python -c  import this    grep  only one  There should be one-- and preferably only one --obvious way to do it

User · Answer

Python is a pragmatic programming language  and the reasons for len   being a function and not a method of str  list  dict etc  are pragmatic   The len   built-in function deals directly with built-in types  the CPython implementation of len   actually returns the value of the ob size field in the PyVarObject C struct that represents any variable-sized built-in object in memory  This is much faster than calling a method -- no attribute lookup needs to happen  Getting the number of items in a collection is a common operation and must work efficiently for such basic and diverse types as str  list  array array etc    However  to promote consistency  when applying len o  to a user-defined type  Python calls o   len     as a fallback     len      abs   and all the other special methods documented in the Python Data Model make it easy to create objects that behave like the built-ins  enabling the expressive and highly consistent APIs we call  Pythonic     By implementing special methods your objects can support iteration  overload infix operators  manage contexts in with blocks etc  You can think of the Data Model as a way of using the Python language itself as a framework where the objects you create can be integrated seamlessly   A second reason  supported by quotes from Guido van Rossum like this one  is that it is easier to read and write len s  than s len     The notation len s  is consistent with unary operators with prefix notation  like abs n   len   is used way more often than abs    and it deserves to be as easy to write    There may also be a historical reason  in the ABC language which preceded Python  and was very influential in its design   there was a unary operator written as  s which meant len s

User · Answer

There are some great answers here  and so before I give my own I d like to highlight a few of the gems  no ruby pun intended  I ve read here    Python is not a pure OOP language -- it s a general purpose  multi-paradigm language that allows the programmer to use the paradigm they are most comfortable with and or the paradigm that is best suited for their solution  Python has first-class functions  so len is actually an object  Ruby  on the other hand  doesn t have first class functions  So the len function object has it s own methods that you can inspect by running dir len     If you don t like the way this works in your own code  it s trivial for you to re-implement the containers using your preferred method  see example below     gt  gt  gt  class List list           def len self               return len self       gt  gt  gt  class Dict dict           def len self               return len self       gt  gt  gt  class Tuple tuple           def len self               return len self       gt  gt  gt  class Set set           def len self               return len self       gt  gt  gt  my list   List  1 2 3 4 5 6 7 8 9  A   B   C   D   E   F     gt  gt  gt  my dict   Dict   key    value    site    stackoverflow     gt  gt  gt  my set   Set  1 2 3 4 5 6 7 8 9  A   B   C   D   E   F     gt  gt  gt  my tuple   Tuple  1 2 3 4 5 6 7 8 9  A   B   C   D   E   F     gt  gt  gt  my containers   Tuple  my list  my dict  my set  my tuple    gt  gt  gt   gt  gt  gt  for container in my containers          print container len       15 2 15 15

User · Answer

Jim s answer to this question may help  I copy it here  Quoting Guido van Rossum   First of all  I chose len x  over x len   for HCI reasons  def   len     came much later   There are two intertwined reasons actually  both HCI   a  For some operations  prefix notation just reads better than postfix     prefix  and infix   operations have a long tradition in mathematics which likes notations where the visuals help the mathematician thinking about a problem  Compare the easy with which we rewrite a formula like x  a b  into xa   xb to the clumsiness of doing the same thing using a raw OO notation   b  When I read code that says len x  I know that it is asking for the length of something  This tells me two things  the result is an integer  and the argument is some kind of container  To the contrary  when I read x len    I have to already know that x is some kind of container implementing an interface or inheriting from a class that has a standard len    Witness the confusion we occasionally have when a class that is not implementing a mapping has a get   or keys   method  or something that isn   t a file has a write   method  Saying the same thing in another way  I see    len    as a built-in operation  I   d hate to lose that

User · Answer

You can also say    gt  gt  x    test   gt  gt  len x  4   Using Python 2 7 3

User · Answer

met  python -c  import this    grep  only one  There should be one-- and preferably only one --obvious way to do it

User · Answer

Python is a pragmatic programming language  and the reasons for len   being a function and not a method of str  list  dict etc  are pragmatic   The len   built-in function deals directly with built-in types  the CPython implementation of len   actually returns the value of the ob size field in the PyVarObject C struct that represents any variable-sized built-in object in memory  This is much faster than calling a method -- no attribute lookup needs to happen  Getting the number of items in a collection is a common operation and must work efficiently for such basic and diverse types as str  list  array array etc    However  to promote consistency  when applying len o  to a user-defined type  Python calls o   len     as a fallback     len      abs   and all the other special methods documented in the Python Data Model make it easy to create objects that behave like the built-ins  enabling the expressive and highly consistent APIs we call  Pythonic     By implementing special methods your objects can support iteration  overload infix operators  manage contexts in with blocks etc  You can think of the Data Model as a way of using the Python language itself as a framework where the objects you create can be integrated seamlessly   A second reason  supported by quotes from Guido van Rossum like this one  is that it is easier to read and write len s  than s len     The notation len s  is consistent with unary operators with prefix notation  like abs n   len   is used way more often than abs    and it deserves to be as easy to write    There may also be a historical reason  in the ABC language which preceded Python  and was very influential in its design   there was a unary operator written as  s which meant len s

User · Answer

There is a len method    gt  gt  gt  a    a string of some length   gt  gt  gt  a   len     23  gt  gt  gt  a   len    lt method-wrapper    len    of str object at 0x02005650 gt

User · Answer

met  python -c  import this    grep  only one  There should be one-- and preferably only one --obvious way to do it

[python] Why does Python code use len() function instead of a length method?

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