Why is it string join list instead of list join string

Question

This has always confused me  It seems like this would be nicer   my list     Hello    world   print my list join  -      Produce   Hello-world    Than this   my list     Hello    world   print  -  join my list     Produce   Hello-world    Is there a specific reason it is like this

User · Answer

This was discussed in the String methods... finally thread in the Python-Dev achive, and was accepted by Guido. This thread began in Jun 1999, and str.join was included in Python 1.6 which was released in Sep 2000 (and supported Unicode). Python 2.0 (supported str methods including join) was released in Oct 2000.

There were four options proposed in this thread:
- str.join(seq)
- seq.join(str)
- seq.reduce(str)
- join as a built-in function
Guido wanted to support not only lists, tuples, but all sequences/iterables.
seq.reduce(str) is difficult for new-comers.
seq.join(str) introduces unexpected dependency from sequences to str/unicode.
join() as a built-in function would support only specific data types. So using a built in namespace is not good. If join() supports many datatypes, creating optimized implementation would be difficult, if implemented using the __add__ method then it's O(n²).
The separator string (sep) should not be omitted. Explicit is better than implicit.

There are no other reasons offered in this thread.

Here are some additional thoughts (my own, and my friend's):

Unicode support was coming, but it was not final. At that time UTF-8 was the most likely about to replace UCS2/4. To calculate total buffer length of UTF-8 strings it needs to know character coding rule.
At that time, Python had already decided on a common sequence interface rule where a user could create a sequence-like (iterable) class. But Python didn't support extending built-in types until 2.2. At that time it was difficult to provide basic iterable class (which is mentioned in another comment).

Guido's decision is recorded in a historical mail, deciding on str.join(seq):

Funny, but it does seem right! Barry, go for it...
--Guido van Rossum

User · Answer

Primarily because the result of a someString join   is a string   The sequence  list or tuple or whatever  doesn t appear in the result  just a string   Because the result is a string  it makes sense as a method of a string

User · Answer

This was discussed in the String methods... finally thread in the Python-Dev achive, and was accepted by Guido. This thread began in Jun 1999, and str.join was included in Python 1.6 which was released in Sep 2000 (and supported Unicode). Python 2.0 (supported str methods including join) was released in Oct 2000.

There were four options proposed in this thread:
- str.join(seq)
- seq.join(str)
- seq.reduce(str)
- join as a built-in function
Guido wanted to support not only lists, tuples, but all sequences/iterables.
seq.reduce(str) is difficult for new-comers.
seq.join(str) introduces unexpected dependency from sequences to str/unicode.
join() as a built-in function would support only specific data types. So using a built in namespace is not good. If join() supports many datatypes, creating optimized implementation would be difficult, if implemented using the __add__ method then it's O(n²).
The separator string (sep) should not be omitted. Explicit is better than implicit.

There are no other reasons offered in this thread.

Here are some additional thoughts (my own, and my friend's):

Unicode support was coming, but it was not final. At that time UTF-8 was the most likely about to replace UCS2/4. To calculate total buffer length of UTF-8 strings it needs to know character coding rule.
At that time, Python had already decided on a common sequence interface rule where a user could create a sequence-like (iterable) class. But Python didn't support extending built-in types until 2.2. At that time it was difficult to provide basic iterable class (which is mentioned in another comment).

Guido's decision is recorded in a historical mail, deciding on str.join(seq):

Funny, but it does seem right! Barry, go for it...
--Guido van Rossum

User · Answer

I agree that it s counterintuitive at first  but there s a good reason  Join can t be a method of a list because    it must work for different iterables too  tuples  generators  etc    it must have different behavior between different types of strings    There are actually two join methods  Python 3 0     gt  gt  gt  b   join  lt built-in method join of bytes object at 0x00A46800 gt   gt  gt  gt     join  lt built-in method join of str object at 0x00A28D40 gt    If join was a method of a list  then it would have to inspect its arguments to decide which one of them to call  And you can t join byte and str together  so the way they have it now makes sense

User · Answer

Both are not nice   string join xs  delimit  means that the string module is aware of the existence of a list  which it has no business knowing about  since the string module only works with strings   list join delimit  is a bit nicer because we re so used to strings being a fundamental type and lingually speaking  they are   However this means that join needs to be dispatched dynamically because in the arbitrary context of a split   n   the python compiler might not know what a is  and will need to look it up analogously to vtable lookup   which is expensive if you do it a lot of times   if the python runtime compiler knows that list is a built in module  it can skip the dynamic lookup and encode the intent into the bytecode directly  whereas otherwise it needs to dynamically resolve  join  of  a   which may be up several layers of inheritence per call since between calls  the meaning of join may have changed  because python is a dynamic language    sadly  this is the ultimate flaw of abstraction  no matter what abstraction you choose  your abstraction will only make sense in the context of the problem you re trying to solve  and as such you can never have a consistent abstraction that doesn t become inconsistent with underlying ideologies as you start gluing them together without wrapping them in a view that is consistent with your ideology  Knowing this  python s approach is more flexible since it s cheaper  it s up to you to pay more to make it look  nicer   either by making your own wrapper  or your own preprocessor

User · Answer

Why is it string join list  instead of list join string     This is because join is a  string  method  It creates a string from any iterable  If we stuck the method on lists  what about when we have iterables that aren t lists    What if you have a tuple of strings  If this were a list method  you would have to cast every such iterator of strings as a list before you could join the elements into a single string  For example   some strings     foo    bar    baz     Let s roll our own list join method   class OurList list        def join self  s           return s join self    And to use it  note that we have to first create a list from each iterable to join the strings in that iterable  wasting both memory and processing power    gt  gt  gt  l   OurList some strings    step 1  create our list  gt  gt  gt  l join         step 2  use our list join method   foo  bar  baz    So we see we have to add an extra step to use our list method  instead of just using the builtin string method    gt  gt  gt        join some strings    a single step   foo   bar   baz    Performance Caveat for Generators  The algorithm Python uses to create the final string with str join actually has to pass over the iterable twice  so if you provide it a generator expression  it has to materialize it into a list first before it can create the final string    Thus  while passing around generators is usually better than list comprehensions  str join is an exception    gt  gt  gt  import timeit  gt  gt  gt  min timeit repeat lambda     join str i  for i in range 10  if i    3 839168446022086  gt  gt  gt  min timeit repeat lambda     join  str i  for i in range 10  if i     3 339879313018173   Nevertheless  the str join operation is still semantically a  string  operation  so it still makes sense to have it on the str object than on miscellaneous iterables

User · Answer

It s because any iterable can be joined  e g  list  tuple  dict  set   but its contents and the  quot joiner quot  must be strings  For example      join   welcome    to    stack    overflow        join   welcome    to    stack    overflow      welcome to stack overflow   Using something other than strings will raise the following error  TypeError  sequence item 0  expected str instance  int found

User · Answer

Because the join   method is in the string class  instead of the list class  I agree it looks funny  See http   www faqs org docs diveintopython odbchelper join html   Historical note  When I first learned Python  I expected join to be a method of a list  which would take the delimiter as an argument  Lots of people feel the same way  and there   s a story behind the join method  Prior to Python 1 6  strings didn   t have all these useful methods  There was a separate string module which contained all the string functions  each function took a string as its first argument  The functions were deemed important enough to put onto the strings themselves  which made sense for functions like lower  upper  and split  But many hard-core Python programmers objected to the new join method  arguing that it should be a method of the list instead  or that it shouldn   t move at all but simply stay a part of the old string module  which still has lots of useful stuff in it   I use the new join method exclusively  but you will see code written either way  and if it really bothers you  you can use the old string join function instead  --- Mark Pilgrim  Dive into Python

User · Answer

Think of it as the natural orthogonal operation to split   I understand why it is applicable to anything iterable and so can t easily be implemented just on list   For readability  I d like to see it in the language but I don t think that is actually feasible - if iterability were an interface then it could be added to the interface but it is just a convention and so there s no central way to add it to the set of things which are iterable

User · Answer

Think of it as the natural orthogonal operation to split   I understand why it is applicable to anything iterable and so can t easily be implemented just on list   For readability  I d like to see it in the language but I don t think that is actually feasible - if iterability were an interface then it could be added to the interface but it is just a convention and so there s no central way to add it to the set of things which are iterable

User · Answer

Both are not nice   string join xs  delimit  means that the string module is aware of the existence of a list  which it has no business knowing about  since the string module only works with strings   list join delimit  is a bit nicer because we re so used to strings being a fundamental type and lingually speaking  they are   However this means that join needs to be dispatched dynamically because in the arbitrary context of a split   n   the python compiler might not know what a is  and will need to look it up analogously to vtable lookup   which is expensive if you do it a lot of times   if the python runtime compiler knows that list is a built in module  it can skip the dynamic lookup and encode the intent into the bytecode directly  whereas otherwise it needs to dynamically resolve  join  of  a   which may be up several layers of inheritence per call since between calls  the meaning of join may have changed  because python is a dynamic language    sadly  this is the ultimate flaw of abstraction  no matter what abstraction you choose  your abstraction will only make sense in the context of the problem you re trying to solve  and as such you can never have a consistent abstraction that doesn t become inconsistent with underlying ideologies as you start gluing them together without wrapping them in a view that is consistent with your ideology  Knowing this  python s approach is more flexible since it s cheaper  it s up to you to pay more to make it look  nicer   either by making your own wrapper  or your own preprocessor

User · Answer

I agree that it s counterintuitive at first  but there s a good reason  Join can t be a method of a list because    it must work for different iterables too  tuples  generators  etc    it must have different behavior between different types of strings    There are actually two join methods  Python 3 0     gt  gt  gt  b   join  lt built-in method join of bytes object at 0x00A46800 gt   gt  gt  gt     join  lt built-in method join of str object at 0x00A28D40 gt    If join was a method of a list  then it would have to inspect its arguments to decide which one of them to call  And you can t join byte and str together  so the way they have it now makes sense

User · Answer

Because the join   method is in the string class  instead of the list class  I agree it looks funny  See http   www faqs org docs diveintopython odbchelper join html   Historical note  When I first learned Python  I expected join to be a method of a list  which would take the delimiter as an argument  Lots of people feel the same way  and there   s a story behind the join method  Prior to Python 1 6  strings didn   t have all these useful methods  There was a separate string module which contained all the string functions  each function took a string as its first argument  The functions were deemed important enough to put onto the strings themselves  which made sense for functions like lower  upper  and split  But many hard-core Python programmers objected to the new join method  arguing that it should be a method of the list instead  or that it shouldn   t move at all but simply stay a part of the old string module  which still has lots of useful stuff in it   I use the new join method exclusively  but you will see code written either way  and if it really bothers you  you can use the old string join function instead  --- Mark Pilgrim  Dive into Python

User · Answer

It s because any iterable can be joined  e g  list  tuple  dict  set   but its contents and the  quot joiner quot  must be strings  For example      join   welcome    to    stack    overflow        join   welcome    to    stack    overflow      welcome to stack overflow   Using something other than strings will raise the following error  TypeError  sequence item 0  expected str instance  int found

User · Answer

I agree that it s counterintuitive at first  but there s a good reason  Join can t be a method of a list because    it must work for different iterables too  tuples  generators  etc    it must have different behavior between different types of strings    There are actually two join methods  Python 3 0     gt  gt  gt  b   join  lt built-in method join of bytes object at 0x00A46800 gt   gt  gt  gt     join  lt built-in method join of str object at 0x00A28D40 gt    If join was a method of a list  then it would have to inspect its arguments to decide which one of them to call  And you can t join byte and str together  so the way they have it now makes sense

User · Answer

Think of it as the natural orthogonal operation to split   I understand why it is applicable to anything iterable and so can t easily be implemented just on list   For readability  I d like to see it in the language but I don t think that is actually feasible - if iterability were an interface then it could be added to the interface but it is just a convention and so there s no central way to add it to the set of things which are iterable

User · Answer

Why is it string join list  instead of list join string     This is because join is a  string  method  It creates a string from any iterable  If we stuck the method on lists  what about when we have iterables that aren t lists    What if you have a tuple of strings  If this were a list method  you would have to cast every such iterator of strings as a list before you could join the elements into a single string  For example   some strings     foo    bar    baz     Let s roll our own list join method   class OurList list        def join self  s           return s join self    And to use it  note that we have to first create a list from each iterable to join the strings in that iterable  wasting both memory and processing power    gt  gt  gt  l   OurList some strings    step 1  create our list  gt  gt  gt  l join         step 2  use our list join method   foo  bar  baz    So we see we have to add an extra step to use our list method  instead of just using the builtin string method    gt  gt  gt        join some strings    a single step   foo   bar   baz    Performance Caveat for Generators  The algorithm Python uses to create the final string with str join actually has to pass over the iterable twice  so if you provide it a generator expression  it has to materialize it into a list first before it can create the final string    Thus  while passing around generators is usually better than list comprehensions  str join is an exception    gt  gt  gt  import timeit  gt  gt  gt  min timeit repeat lambda     join str i  for i in range 10  if i    3 839168446022086  gt  gt  gt  min timeit repeat lambda     join  str i  for i in range 10  if i     3 339879313018173   Nevertheless  the str join operation is still semantically a  string  operation  so it still makes sense to have it on the str object than on miscellaneous iterables

User · Answer

The variables my list and  -  are both objects  Specifically  they re instances of the classes list and str  respectively  The join function belongs to the class str  Therefore  the syntax  -  join my list  is used because the object  -  is taking my list as an input

User · Answer

I agree that it s counterintuitive at first  but there s a good reason  Join can t be a method of a list because    it must work for different iterables too  tuples  generators  etc    it must have different behavior between different types of strings    There are actually two join methods  Python 3 0     gt  gt  gt  b   join  lt built-in method join of bytes object at 0x00A46800 gt   gt  gt  gt     join  lt built-in method join of str object at 0x00A28D40 gt    If join was a method of a list  then it would have to inspect its arguments to decide which one of them to call  And you can t join byte and str together  so the way they have it now makes sense

User · Answer

Primarily because the result of a someString join   is a string   The sequence  list or tuple or whatever  doesn t appear in the result  just a string   Because the result is a string  it makes sense as a method of a string

User · Answer

Because the join   method is in the string class  instead of the list class  I agree it looks funny  See http   www faqs org docs diveintopython odbchelper join html   Historical note  When I first learned Python  I expected join to be a method of a list  which would take the delimiter as an argument  Lots of people feel the same way  and there   s a story behind the join method  Prior to Python 1 6  strings didn   t have all these useful methods  There was a separate string module which contained all the string functions  each function took a string as its first argument  The functions were deemed important enough to put onto the strings themselves  which made sense for functions like lower  upper  and split  But many hard-core Python programmers objected to the new join method  arguing that it should be a method of the list instead  or that it shouldn   t move at all but simply stay a part of the old string module  which still has lots of useful stuff in it   I use the new join method exclusively  but you will see code written either way  and if it really bothers you  you can use the old string join function instead  --- Mark Pilgrim  Dive into Python

User · Answer

- in  quot - quot  join my list  declares that you are converting to a string from joining elements a list It s result-oriented   just for easy memory and understanding  I made an exhaustive cheatsheet of methods of string for your reference  string methods 44          convert     join   split    rsplit   splitlines    partition    rpartition         edit     replace    lstrip    rstrip    strip         search     endswith    startswith    count    index    find   rindex    rfind          condition     isalnum    isalpha    isdecimal    isdigit    isnumeric   isidentifier                      islower   istitle    isupper   isprintable    isspace           text     lower    upper    capitalize    title    swapcase                 center    ljust    rjust    zfill    expandtabs   casefold         encode     translate    maketrans    encode         format     format    format map

User · Answer

Because the join   method is in the string class  instead of the list class  I agree it looks funny  See http   www faqs org docs diveintopython odbchelper join html   Historical note  When I first learned Python  I expected join to be a method of a list  which would take the delimiter as an argument  Lots of people feel the same way  and there   s a story behind the join method  Prior to Python 1 6  strings didn   t have all these useful methods  There was a separate string module which contained all the string functions  each function took a string as its first argument  The functions were deemed important enough to put onto the strings themselves  which made sense for functions like lower  upper  and split  But many hard-core Python programmers objected to the new join method  arguing that it should be a method of the list instead  or that it shouldn   t move at all but simply stay a part of the old string module  which still has lots of useful stuff in it   I use the new join method exclusively  but you will see code written either way  and if it really bothers you  you can use the old string join function instead  --- Mark Pilgrim  Dive into Python

User · Answer

It s because any iterable can be joined  e g  list  tuple  dict  set   but its contents and the  quot joiner quot  must be strings  For example      join   welcome    to    stack    overflow        join   welcome    to    stack    overflow      welcome to stack overflow   Using something other than strings will raise the following error  TypeError  sequence item 0  expected str instance  int found

User · Answer

Primarily because the result of a someString join   is a string   The sequence  list or tuple or whatever  doesn t appear in the result  just a string   Because the result is a string  it makes sense as a method of a string

User · Answer

Primarily because the result of a someString join   is a string   The sequence  list or tuple or whatever  doesn t appear in the result  just a string   Because the result is a string  it makes sense as a method of a string

User · Answer

Think of it as the natural orthogonal operation to split   I understand why it is applicable to anything iterable and so can t easily be implemented just on list   For readability  I d like to see it in the language but I don t think that is actually feasible - if iterability were an interface then it could be added to the interface but it is just a convention and so there s no central way to add it to the set of things which are iterable

User · Answer

- in  quot - quot  join my list  declares that you are converting to a string from joining elements a list It s result-oriented   just for easy memory and understanding  I made an exhaustive cheatsheet of methods of string for your reference  string methods 44          convert     join   split    rsplit   splitlines    partition    rpartition         edit     replace    lstrip    rstrip    strip         search     endswith    startswith    count    index    find   rindex    rfind          condition     isalnum    isalpha    isdecimal    isdigit    isnumeric   isidentifier                      islower   istitle    isupper   isprintable    isspace           text     lower    upper    capitalize    title    swapcase                 center    ljust    rjust    zfill    expandtabs   casefold         encode     translate    maketrans    encode         format     format    format map

User · Answer

The variables my list and  -  are both objects  Specifically  they re instances of the classes list and str  respectively  The join function belongs to the class str  Therefore  the syntax  -  join my list  is used because the object  -  is taking my list as an input

User · Answer

It s because any iterable can be joined  e g  list  tuple  dict  set   but its contents and the  quot joiner quot  must be strings  For example      join   welcome    to    stack    overflow        join   welcome    to    stack    overflow      welcome to stack overflow   Using something other than strings will raise the following error  TypeError  sequence item 0  expected str instance  int found

[python] Why is it string.join(list) instead of list.join(string)?

Examples related to python

Examples related to string

Examples related to list