diff options
Diffstat (limited to 'Doc/library')
| -rw-r--r-- | Doc/library/stdtypes.rst | 21 |
1 files changed, 12 insertions, 9 deletions
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst index cdb2a4a..5bb4324 100644 --- a/Doc/library/stdtypes.rst +++ b/Doc/library/stdtypes.rst @@ -968,15 +968,18 @@ Notes: If *k* is ``None``, it is treated like ``1``. (6) - .. impl-detail:: - - If *s* and *t* are both strings, some Python implementations such as - CPython can usually perform an in-place optimization for assignments of - the form ``s = s + t`` or ``s += t``. When applicable, this optimization - makes quadratic run-time much less likely. This optimization is both - version and implementation dependent. For performance sensitive code, it - is preferable to use the :meth:`str.join` method which assures consistent - linear concatenation performance across versions and implementations. + Concatenating immutable strings always results in a new object. This means + that building up a string by repeated concatenation will have a quadratic + runtime cost in the total string length. To get a linear runtime cost, + you must switch to one of the alternatives below: + + * if concatenating :class:`str` objects, you can build a list and use + :meth:`str.join` at the end; + + * if concatenating :class:`bytes` objects, you can similarly use + :meth:`bytes.join`, or you can do in-place concatenation with a + :class:`bytearray` object. :class:`bytearray` objects are mutable and + have an efficient overallocation mechanism. .. _string-methods: |