[Python-Dev] Question about the current implementation of str

Victor Stinner victor.stinner at gmail.com
Sat Apr 9 03:52:24 EDT 2016

Le 9 avr. 2016 03:04, "Larry Hastings" <larry at hastings.org> a écrit :
> Although the str object is immutable from Python's perspective, the C
object itself is mutable.  For example, for dynamically-created strings the
hash field may be lazy-computed and cached inside the object.

Yes, the hash is computed once on demand. It doesn't matter how you build
the string.

> I was wondering if there were other fields like this.  For example, are
there similar lazy-computed cached objects for the different encoded
versions (utf8 utf16) of the str?

Cached utf8 is only cached when you call the C functions filling this
cache. The Python str.encode('utf8') doesn't fill the cache, but it uses it.

On Windows, there is a cache for wchar_t* which is utf16. This format is
used by all C functions of the Windows API (Python should only use the
Unicode flavor of the Windows API).

I don't recall other caches.

> What would really help an exhaustive list of the fields of a str object
that may ever change after the object's initial creation.

I don't recall exactly what happens if a cache is created and then the
string is modified. If I recall correctly, the cache is invalidated.

But the hash is used as an heuristic to decide if a string is "immutable"
or not, the refcount is also used by the heuristic. If the string is
immutable, an operation like resize must create a new string.

You can document the PEP 393 in Include/unicodeobject.h.

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