How do I append one string to another in Python

Question

I want an efficient way to append one string to another in Python  other than the following   var1    foo  var2    bar  var3   var1   var2   Is there any good built-in method to use

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it really depends on your application  If you re looping through hundreds of words and want to append them all into a list   join   is better  But if you re putting together a long sentence  you re better off using

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If you only have one reference to a string and you concatenate another string to the end  CPython now special cases this and tries to extend the string in place   The end result is that the operation is amortized O n    e g   s      for i in range n       s  str i    used to be O n 2   but now it is O n    From the source  bytesobject c    void PyBytes ConcatAndDel register PyObject   pv  register PyObject  w        PyBytes Concat pv  w       Py XDECREF w          The following function breaks the notion that strings are immutable     it changes the size of a string   We get away with this only if there    is only one module referencing the object   You can also think of it    as creating a new string object and destroying the old one  only    more efficiently   In any case  don t use this if the string may    already be known to some other part of the code       Note that if there s not enough memory to resize the string  the original    string object at  pv is deallocated   pv is set to NULL  an  out of    memory  exception is set  and -1 is returned   Else  on success  0 is    returned  and the value in  pv may or may not be the same as on input     As always  an extra byte is allocated for a trailing  0 byte  newsize    does  not  include that   and a trailing  0 byte is stored      int  PyBytes Resize PyObject   pv  Py ssize t newsize        register PyObject  v      register PyBytesObject  sv      v    pv      if   PyBytes Check v     Py REFCNT v     1    newsize  lt  0             pv   0          Py DECREF v           PyErr BadInternalCall            return -1               XXX UNREF NEWREF interface should be more symmetrical         Py DEC REFTOTAL       Py ForgetReference v        pv    PyObject            PyObject REALLOC  char   v  PyBytesObject SIZE   newsize       if   pv    NULL            PyObject Del v           PyErr NoMemory            return -1             Py NewReference  pv       sv    PyBytesObject     pv      Py SIZE sv    newsize      sv- gt ob sval newsize      0       sv- gt ob shash   -1              invalidate cached hash value        return 0      It s easy enough to verify empirically      python -m timeit -s s      for i in xrange 10  s   a   1000000 loops  best of 3  1 85 usec per loop   python -m timeit -s s      for i in xrange 100  s   a   10000 loops  best of 3  16 8 usec per loop   python -m timeit -s s      for i in xrange 1000  s   a   10000 loops  best of 3  158 usec per loop   python -m timeit -s s      for i in xrange 10000  s   a   1000 loops  best of 3  1 71 msec per loop   python -m timeit -s s      for i in xrange 100000  s   a   10 loops  best of 3  14 6 msec per loop   python -m timeit -s s      for i in xrange 1000000  s   a   10 loops  best of 3  173 msec per loop   It s important however to note that this optimisation isn t part of the Python spec  It s only in the cPython implementation as far as I know  The same empirical testing on pypy or jython for example might show the older O n  2  performance       pypy -m timeit -s s      for i in xrange 10  s   a   10000 loops  best of 3  90 8 usec per loop   pypy -m timeit -s s      for i in xrange 100  s   a   1000 loops  best of 3  896 usec per loop   pypy -m timeit -s s      for i in xrange 1000  s   a   100 loops  best of 3  9 03 msec per loop   pypy -m timeit -s s      for i in xrange 10000  s   a   10 loops  best of 3  89 5 msec per loop   So far so good  but then      pypy -m timeit -s s      for i in xrange 100000  s   a   10 loops  best of 3  12 8 sec per loop   ouch even worse than quadratic  So pypy is doing something that works well with short strings  but performs poorly for larger strings

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str1    Hello  str2    World  newstr       join  str1  str2     That joins str1 and str2 with a space as separators  You can also do    join str1  str2        str join   takes an iterable  so you d have to put the strings in a list or a tuple   That s about as efficient as it gets for a builtin method

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If you need to do many append operations to build a large string  you can use StringIO or cStringIO  The interface is like a file  ie  you write to append text to it   If you re just appending two strings then just use

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append strings with add function str1    quot Hello quot  str2    quot  World quot  str3   str   add   str2  print str3   Output Hello World

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Python 3 6 gives us f-strings  which are a delight   var1    foo  var2    bar  var3   f  var1  var2   print var3                          prints foobar   You can do most anything inside the curly braces  print f 1   1     1   1             prints 1   1    2

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Don t   That is  for most cases you are better off generating the whole string in one go rather then appending to an existing string   For example  don t do  obj1 name         str obj1 count   Instead  use   s  d     obj1 name  obj1 count   That will be easier to read and more efficient

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Basically  no difference  The only consistent trend is that Python seems to be getting slower with every version          List    timeit x      for i in range 100000000      xrange on Python 2 7     x append  a   x      join x    Python 2 7     1 loop  best of 3  7 34 s per loop   Python 3 4     1 loop  best of 3  7 99 s per loop   Python 3 5     1 loop  best of 3  8 48 s per loop   Python 3 6     1 loop  best of 3  9 93 s per loop     String    timeit x      for i in range 100000000      xrange on Python 2 7     x     a    Python 2 7       1 loop  best of 3  7 41 s per loop   Python 3 4     1 loop  best of 3  9 08 s per loop   Python 3 5     1 loop  best of 3  8 82 s per loop   Python 3 6     1 loop  best of 3  9 24 s per loop

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Don t prematurely optimize  If you have no reason to believe there s a speed bottleneck caused by string concatenations then just stick with   and      s     foo  s     bar  s     baz    That said  if you re aiming for something like Java s StringBuilder  the canonical Python idiom is to add items to a list and then use str join to concatenate them all at the end   l      l append  foo   l append  bar   l append  baz    s      join l

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a  foo  b  baaz   a   add   b   out   foobaaz

[python] How do I append one string to another in Python?

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