Python append vs operator on lists why do these give different results

Question

Why do these two operations  append   resp     give different results    gt  gt  gt  c    1  2  3   gt  gt  gt  c  1  2  3   gt  gt  gt  c    c  gt  gt  gt  c  1  2  3  1  2  3   gt  gt  gt  c    1  2  3   gt  gt  gt  c append c   gt  gt  gt  c  1  2  3          gt  gt  gt     In the last case there s actually an infinite recursion  c -1  and c are the same  Why is it different with the   operation

User · Answer

See the documentation      list append x          Add an item to the end of the list  equivalent to a len a       x           list extend L     - Extend the list by appending all the items in the given list    equivalent to a len a      L    c append c   appends  c to itself as an element  Since a list is a reference type  this creates a recursive data structure   c    c is equivalent to extend c   which appends the elements of c to c

User · Answer

you should use extend     gt  gt  gt  c  1 2 3   gt  gt  gt  c extend c   gt  gt  gt  c  1  2  3  1  2  3    other info  append vs  extend

User · Answer

The concatenation operator   is a binary infix operator which  when applied to lists  returns a new list containing all the elements of each of its two operands  The list append   method is a mutator on list which appends its single object argument  in your specific example the list c  to the subject list  In your example this results in c appending a reference to itself  hence the infinite recursion   An alternative to     concatenation The list extend   method is also a mutator method which concatenates its sequence argument with the subject list  Specifically  it appends each of the elements of sequence in iteration order  An aside Being an operator    returns the result of the expression as a new value  Being a non-chaining mutator method  list extend   modifies the subject list in-place and returns nothing  Arrays I ve added this due to the potential confusion which the Abel s answer above may cause by mixing the discussion of lists  sequences and arrays  Arrays were added to Python after sequences and lists  as a more efficient way of storing arrays of integral data types  Do not confuse arrays with lists  They are not the same  From the array docs  Arrays are sequence types and behave very much like lists  except that the type of objects stored in them is constrained  The type is specified at object creation time by using a type code  which is a single character

User · Answer

Python lists are heterogeneous that is the elements in the same list can be any type of object  The expression  c append c  appends the object c what ever it may be to the list  In the case it makes the list itself a member of the list   The expression c    c adds two lists together and assigns the result to the variable c  The overloaded   operator is defined on lists to create a new list whose contents are the elements in the first list and the elements in the second list   So these are really just different expressions used to do different things by design

User · Answer

To explain  quot why quot   The   operation adds the array elements to the original array  The array append operation inserts the array  or any object  into the end of the original array  which results in a reference to self in that spot  hence the infinite recursion   The difference here is that the   operation acts specific when you add an array  it s overloaded like others  see this chapter on sequences  by concatenating the element  The append-method however does literally what you ask  append the object on the right-hand side that you give it  the array or any other object   instead of taking its elements  An alternative Use extend   if you want to use a function that acts similar to the   operator  as others have shown here as well   It s not wise to do the opposite  to try to mimic append with the   operator for lists  see my earlier link on why   Little history For fun  a little history  the birth of the array module in Python in February 1993  it might surprise you  but arrays were added way after sequences and lists came into existence

User · Answer

The method you re looking for is extend    From the Python documentation   list append x      Add an item to the end of the list  equivalent to a len a       x    list extend L      Extend the list by appending all the items in the given list  equivalent to a len a      L   list insert i  x      Insert an item at a given position  The first argument is the index of the element before which to insert  so a insert 0  x  inserts at the front of the list  and a insert len a   x  is equivalent to a append x

User · Answer

append is appending an element to a list  if you want to extend the list with the new list you need to use extend    gt  gt  gt  c    1  2  3   gt  gt  gt  c extend c   gt  gt  gt  c  1  2  3  1  2  3

[python] Python append() vs. + operator on lists, why do these give different results?

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