> On 9 Mar 2023, at 00:37, Rob Cliffe via Python-ideas
> <[email protected]> wrote:
>
> Having had my last proposal shot down in flames, up I bob with another. 😁
See this discussion that has a nice solution proposed with the concat function.
Barry
> It seems to me that it would be useful to be able to make the str.join()
> function put separators, not only between the items of its operand, but also
> optionally at the beginning or end.
> E.g. '|'.join(('Spam', 'Ham', 'Eggs')) returns
> 'Spam|Ham|Eggs'
> but it might be useful to make it return one of
> '|Spam|Ham|Eggs'
> 'Spam|Ham|Eggs|'
> '|Spam|Ham|Eggs|'
> Again, I suggest that this apply to byte strings as well as strings.
> Going through the 3.8.3 stdlib I have found
> 24 examples where the separator needs to be added at the beginning
> 52 where the separator needs to be added at the end
> 4 where the separator needs to be added at the both ends
> I list these examples below. Apologies if there are any mistakes.
>
> While guessing is no substitute for measurement, it seems plausible that
> using this feature
> where appropriate would increase runtime performance by avoiding 1 (or 2)
> calls of str.__add__.
> This is perhaps more relevant when the separator is not a short constant
> string,
> as in this example:
> Lib\email\_header_value_parser.py:2854: return
> policy.linesep.join(lines) + policy.linesep
> Note also this example:
> Lib\site-packages\setuptools\command\build_ext.py:221: pkg =
> '.'.join(ext._full_name.split('.')[:-1] + [''])
> where the author has used the unintuitive device of appending an empty string
> to a list to force join() to add an extra final dot, thereby avoiding 1 call
> of str.__add__
> at the cost of 1 call of list.append.
>
> What I cannot decide is what the best API would be.
> str.join() currently takes only 1 parameter, so it would be possible to add
> an extra parameter or two.
> One scheme would be to have an atEnds parameter which could take values such
> as
> 0=default behaviour 1=add sep at start 2=add sep at end 3=add sep at
> both ends
> or
> 's'=add sep at start 'e'=add sep at end 'b'=add sep at both ends
> (some) other=default behaviour
> Another would be to have 2 parameters, atStart and atEnd, which would both
> default to False or 0. E.g.
> '|'.join(('Spam', 'Ham', 'Eggs'), 1) == '|Spam|Ham|Eggs'
> '|'.join(('Spam', 'Ham', 'Eggs'), 0, 1) == 'Spam|Ham|Eggs|'
> Neither scheme results in particularly transparent usage, though no worse than
> s.splitlines(True) # What on earth is this parameter???
>
> Corner case:
> What if join() is passed an empty sequence? This is debatable,
> but I think it should return the separator if requested to add it
> at the beginning or end, and double it up if both are requested.
> This would preserve identities such as
> sep.join(seq, <PleaseAddSeparatorAtBeginning>) == sep + sep.join(seq)
>
> Best wishes
> Rob Cliffe
>
> EXAMPLES WHERE SEPARATOR ADDED AT START:
>
> Lib\http\server.py:933: splitpath = ('/' + '/'.join(head_parts), tail_part)
> Lib\site-packages\numpy\ctypeslib.py:333: name += "_"+"_".join(flags)
> Lib\site-packages\numpy\testing\_private\utils.py:842: err_msg += '\n' +
> '\n'.join(remarks)
> Lib\site-packages\pip\_vendor\pyparsing\core.py:2092-2095:
> out = [
> "\n" + "\n".join(comments) if comments else "",
> pyparsing_test.with_line_numbers(t) if with_line_numbers else
> t,
> ]
> Lib\site-packages\pip\_vendor\requests\status_codes.py:121-125:
> __doc__ = (
> __doc__ + "\n" + "\n".join(doc(code) for code in sorted(_codes))
> if __doc__ is not None
> else None
> )
> Lib\site-packages\reportlab\lib\utils.py:1093: self._writeln(' '+'
> '.join(A.__self__))
> Lib\site-packages\reportlab\platypus\flowables.py:708: L =
> "\n"+"\n".join(L)
> Lib\site-packages\twisted\mail\smtp.py:1647: r.append(c + b' ' + b'
> '.join(v))
> Lib\site-packages\twisted\protocols\ftp.py:1203: return (PWD_REPLY,
> '/' + '/'.join(self.workingDirectory))
> Lib\site-packages\twisted\runner\procmon.py:424-426:
> return ('<' + self.__class__.__name__ + ' '
> + ' '.join(l)
> + '>')
> Lib\site-packages\twisted\web\rewrite.py:34: request.path =
> '/'+'/'.join(request.prepath+request.postpath)
> Lib\site-packages\twisted\web\rewrite.py:51: request.path =
> '/'+'/'.join(request.prepath+request.postpath)
> Lib\site-packages\twisted\web\twcgi.py:78: scriptName = b"/" +
> b"/".join(request.prepath)
> Lib\site-packages\twisted\web\twcgi.py:95: env["PATH_INFO"] = "/" +
> "/".join(pp)
> Lib\site-packages\twisted\web\vhost.py:115: request.path = b'/' +
> b'/'.join(request.postpath)
> Lib\site-packages\twisted\web\wsgi.py:283: scriptName = b'/' +
> b'/'.join(request.prepath)
> Lib\site-packages\twisted\web\wsgi.py:288: pathInfo = b'/' +
> b'/'.join(request.postpath)
> Lib\site-packages\twisted\web\test\test_wsgi.py:272: uri = '/' +
> '/'.join([urlquote(seg, safe) for seg in requestSegments])
> Lib\site-packages\wx\py\magic.py:55: command =
> 'sx("'+aliasDict[c[0]]+' '+' '.join(c[1:])+'")'
> Lib\site-packages\zope\interface\exceptions.py:257-260:
> return '\n ' + '\n '.join(
> x._str_details.strip() if isinstance(x, _TargetInvalid) else
> str(x)
> for x in self.exceptions
> )
> Lib\smtplib.py:537 and 545: optionlist = ' ' + ' '.join(options)
> Lib\unittest\case.py:1094-1096:
> diffMsg = '\n' + '\n'.join(
> difflib.ndiff(pprint.pformat(seq1).splitlines(),
> pprint.pformat(seq2).splitlines()))
> Lib\unittest\case.py:1207-1209:
> diff = ('\n' + '\n'.join(difflib.ndiff(
> pprint.pformat(d1).splitlines(),
> pprint.pformat(d2).splitlines())))
>
> SEPARATOR ADDED AT END:
>
> Lib\distutils\command\config.py:303: body = "\n".join(body) + "\n"
> Lib\email\contentmanager.py:145: def embedded_body(lines): return
> linesep.join(lines) + linesep
> Lib\email\contentmanager.py:146: def normal_body(lines): return
> b'\n'.join(lines) + b'\n'
> Lib\email\policy.py:215: return name + ': ' + self.linesep.join(lines)
> + self.linesep
> Lib\email\_header_value_parser.py:2854: return policy.linesep.join(lines)
> + policy.linesep
> Lib\site-packages\numpy\distutils\command\config.py:346: body =
> '\n'.join(body) + "\n"
> Lib\site-packages\numpy\distutils\command\config.py:407: body =
> '\n'.join(body) + "\n"
> Lib\site-packages\oauthlib\oauth2\rfc6749\tokens.py:158: base_string =
> '\n'.join(base) + '\n'
> Lib\site-packages\PIL\ImageCms.py:770: return "\r\n\r\n".join(arr) +
> "\r\n\r\n"
> Lib\site-packages\pip\_internal\operations\freeze.py:254: return
> "\n".join(list(self.comments) + [str(req)]) + "\n"
> Lib\site-packages\pip\_internal\operations\install\legacy.py:54:
> f.write("\n".join(new_lines) + "\n")
> Lib\site-packages\pip\_vendor\pyparsing\testing.py:323-331:
> return (
> header1
> + header2
> + "\n".join(
> "{:{}d}:{}{}".format(i, lineno_width, line, eol_mark)
> for i, line in enumerate(s_lines, start=start_line)
> )
> + "\n"
> )
> Lib\site-packages\pycparser\c_generator.py:117: if n.storage: s += '
> '.join(n.storage) + ' '
> Lib\site-packages\pycparser\c_generator.py:366: if n.funcspec: s = '
> '.join(n.funcspec) + ' '
> Lib\site-packages\pycparser\c_generator.py:367: if n.storage: s += '
> '.join(n.storage) + ' '
> Lib\site-packages\pycparser\c_generator.py:382: if n.quals: s += '
> '.join(n.quals) + ' '
> Lib\site-packages\pycparser\c_generator.py:397: nstr += '
> '.join(modifier.dim_quals) + ' '
> Lib\site-packages\pycparser\c_generator.py:417: return '
> '.join(n.names) + ' '
> Lib\site-packages\pythonwin\pywin\framework\scriptutils.py:109: return
> ".".join(modBits) + "." + fname, newPathReturn
> Lib\site-packages\reportlab\pdfbase\pdfdoc.py:1118: code =
> '\n'.join(code)+'\n'
> Lib\site-packages\reportlab\pdfbase\pdfutils.py:102:
> f.write('\r\n'.join(code)+'\r\n')
> Lib\site-packages\reportlab\pdfbase\_can_cmap_data.py:54: src =
> '\n'.join(buf) + '\n'
> Lib\site-packages\reportlab\pdfgen\pdfimages.py:203: content =
> '\n'.join(self.imageData[3:-1]) + '\n'
> Lib\site-packages\setuptools\command\build_ext.py:221: pkg =
> '.'.join(ext._full_name.split('.')[:-1] + [''])
> Lib\site-packages\setuptools\command\easy_install.py:1056:
> f.write('\n'.join(locals()[name]) + '\n')
> Lib\site-packages\setuptools\command\easy_install.py:1606: data =
> '\n'.join(lines) + '\n'
> Lib\site-packages\setuptools\command\egg_info.py:672:
> cmd.write_file("top-level names", filename, '\n'.join(sorted(pkgs)) + '\n')
> Lib\site-packages\setuptools\command\egg_info.py:683: value =
> '\n'.join(value) + '\n'
> Lib\site-packages\setuptools\_distutils\command\config.py:303: body =
> "\n".join(body) + "\n"
> Lib\site-packages\twisted\conch\manhole.py:360-362:
> return (b'\n'.join(self.interpreter.buffer) +
> b'\n' +
> b''.join(self.lineBuffer))
> Lib\site-packages\twisted\conch\client\knownhosts.py:547-549:
> hostsFileObj.write(
> b"\n".join([entry.toString() for entry in self._added]) +
> b"\n")
> Lib\site-packages\twisted\conch\ssh\keys.py:1340: return
> b'\n'.join(lines) + b'\n'
> Lib\site-packages\twisted\conch\test\test_conch.py:556: expectedResult =
> '\n'.join(['line #%02d' % (i,) for i in range(60)]) + '\n'
> Lib\site-packages\twisted\conch\test\test_helper.py:360:
> self.term.write(b'\n'.join((s1, s2, s3)) + b'\n')
> Lib\site-packages\twisted\internet\test\test_process.py:769:
> scriptFile.write(os.linesep.join(sourceLines) + os.linesep)
> Lib\site-packages\twisted\mail\imap4.py:5713: hdrs = '\r\n'.join(hdrs) +
> '\r\n'
> Lib\site-packages\twisted\mail\imap4.py:5952: base =
> b'.'.join([(x + 1).__bytes__() for x in self.part]) + b'.' + base
> Lib\site-packages\twisted\mail\test\test_pop3.py:312: self.message =
> b'\n'.join(self.lines) + b'\n'
> Lib\site-packages\twisted\mail\test\test_pop3.py:376: output =
> b'\r\n'.join(client.response) + b'\r\n'
> Lib\site-packages\twisted\mail\test\test_smtp.py:100: message =
> b'\n'.join(self.buffer) + b'\n'
> Lib\site-packages\twisted\mail\test\test_smtp.py:344: message =
> b'\n'.join(self.buffer) + b'\n'
> Lib\site-packages\twisted\python\text.py:146: return '\n'.join(lines)+'\n'
> Lib\site-packages\twisted\test\test_iutils.py:40:
> scriptFile.write(os.linesep.join(sourceLines) + os.linesep)
> Lib\site-packages\win32comext\adsi\demos\scp.py:350: description = __doc__
> + "\ncommands:\n" + "\n".join(arg_descs) + "\n"
> Lib\site-packages\wx\py\crust.py:259: self.SetValue('\n'.join(hist) + '\n')
> Lib\site-packages\wx\py\introspect.py:342: command =
> terminator.join(pieces[:-1]) + terminator
> Lib\site-packages\zope\interface\document.py:78: return "\n\n".join(r) +
> "\n\n"
> Lib\test\test_nntplib.py:495: lit = "\r\n".join(lit.splitlines()) +
> "\r\n"
> Lib\test\test_univnewlines.py:24:DATA_LF = "\n".join(DATA_TEMPLATE) + "\n"
> Lib\test\test_univnewlines.py:25:DATA_CR = "\r".join(DATA_TEMPLATE) + "\r"
> Lib\test\test_univnewlines.py:26:DATA_CRLF = "\r\n".join(DATA_TEMPLATE) +
> "\r\n"
> Lib\test\test_tools\test_pindent.py:33: return '\n'.join(line.lstrip()
> for line in data.splitlines()) + '\n'
>
> SEPARATOR ADDED AT BOTH ENDS:
>
> Lib\pydoc.py:1582: sys.stdout.write('\n' +
> '\n'.join(lines[r:r+inc]) + '\n')
> Lib\site-packages\office365\runtime\odata\odata_batch_request.py:129: buffer
> = eol + eol.join(lines) + eol
> Lib\test\test_generators.py:1424: print("|" + "|".join(squares) +
> "|")
> Lib\test\test_generators.py:1620: print("|" + "|".join(row) + "|")
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