<div dir="ltr">This was a CPython idea after all, so I was assuming a C implementation, which means that new flags would have to be added to the string object to denote a string slice, etc.<div><br></div><div>Like I said in another message: it's not that important to me though. I was just curious as to why CPython was designed so that string slicing is linear rather than constant time given that strings are constant.</div><div><br></div><div>I'm getting the impression that the payoff is not worth the complexity.</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Thu, May 7, 2015 at 4:12 PM, Skip Montanaro <span dir="ltr"><<a href="mailto:skip.montanaro@gmail.com" target="_blank">skip.montanaro@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="gmail_extra">I haven't seen anyone else mention it, so I will point out: interoperability with C. In C, strings are NUL-terminated. PyStringObject instances do (or used to) have NUL-terminated strings in them. According to unicodeobject.h, that seems still to be the case:</div><div class="gmail_extra"><br></div><div class="gmail_extra"><div class="gmail_extra"><font face="monospace, monospace">typedef struct {</font></div><div class="gmail_extra"><font face="monospace, monospace"> /* There are 4 forms of Unicode strings:</font></div><div><font face="monospace, monospace"> ...</font></div></div><div class="gmail_extra"><font face="monospace, monospace"> wchar_t *wstr; /* wchar_t representation (<b>null-terminated</b>) */</font></div><div class="gmail_extra"><div class="gmail_extra"><font face="monospace, monospace">} PyASCIIObject;</font></div><div><br></div></div><div class="gmail_extra">and:</div><div class="gmail_extra"><br></div><div class="gmail_extra"><div class="gmail_extra"><font face="monospace, monospace">typedef struct {</font></div><div class="gmail_extra"><font face="monospace, monospace"> PyASCIIObject _base;</font></div><div class="gmail_extra"><font face="monospace, monospace"> Py_ssize_t utf8_length; /* Number of bytes in utf8, <b>excluding the</b></font></div><div class="gmail_extra"><font face="monospace, monospace"><b> * terminating \0</b>. */</font></div><div class="gmail_extra"><font face="monospace, monospace"> char *utf8; /* UTF-8 representation (<b>null-terminated</b>) */</font></div><div class="gmail_extra"><font face="monospace, monospace"> Py_ssize_t wstr_length; /* Number of code points in wstr, possible</font></div><div class="gmail_extra"><font face="monospace, monospace"> * surrogates count as two code points. */</font></div><div class="gmail_extra"><font face="monospace, monospace">} PyCompactUnicodeObject;</font></div><div><br></div></div><div class="gmail_extra">The raw string is NUL-terminated, precisely so copying isn't required in most cases before passing to C. Making s[1:-1] a view onto the underlying string data in s would require you to copy the data when you want to pass the view into C so you could tack on that NUL. That happens a lot, so it's likely you wouldn't save much work, and result in a lot more churn in Python's memory allocator. The only place you could avoid the copy is if the view you are dealing with is a strict suffix of s.</div><span class="HOEnZb"><font color="#888888"><div class="gmail_extra"><br></div><div class="gmail_extra">Skip</div><div class="gmail_extra"><br></div></font></span></div><div class="HOEnZb"><div class="h5">
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