I just spent a few minutes staring at a bug caused by a missing comma
-- I got a mysterious argument count error because instead of foo('a',
'b') I had written foo('a' 'b').
This is a fairly common mistake, and IIRC at Google we even had a lint
rule against this (there was also a Python dialect used for some
specific purpose where this was explicitly forbidden).
Now, with modern compiler technology, we can (and in fact do) evaluate
compile-time string literal concatenation with the '+' operator, so
there's really no reason to support 'a' 'b' any more. (The reason was
always rather flimsy; I copied it from C but the reason why it's
needed there doesn't really apply to Python, as it is mostly useful
inside macros.)
Would it be reasonable to start deprecating this and eventually remove
it from the language?
--
--Guido van Rossum (python.org/~guido)
Stephen J. Turnbull wrote:
> Vernon D. Cole writes:
>
>> I cannot compile a Python extension module with any Microsoft compiler
>> I can obtain.
>
> Your pain is understood, but it's not simple to address it.
FWIW, I'm working on making the compiler easily obtainable. The VS 2008 link that was posted is unofficial, and could theoretically disappear at any time (I'm not in control of that), but the Windows SDK for Windows 7 and .NET 3.5 SP1 (http://www.microsoft.com/en-us/download/details.aspx?id=3138) should be around for as long as Windows 7 is supported. The correct compiler (VC9) is included in this SDK, but unfortunately does not install the vcvarsall.bat file that distutils expects. (Though it's pretty simple to add one that will switch on %1 and call the correct vcvars(86|64|...).bat.)
The SDK needed for Python 3.3 and 3.4 (VC10) is even worse - there are many files missing. I'm hoping we'll be able to set up some sort of downloadable package/tool that will fix this. While we'd obviously love to move CPython onto our latest compilers, it's simply not possible (for good reason). Python 3.4 is presumably locked to VC10, but hopefully 3.5 will be able to use whichever version is current when that decision is made.
> The basic problem is that the ABI changes. Therefore it's going to require
> a complete new set of *all* C extensions for Windows, and the duplication
> of download links for all those extensions from quite a few different vendors
> is likely to confuse a lot of users.
Specifically, the CRT changes. The CRT is an interesting mess of data structures that are exposed in header files, which means while you can have multiple CRTs loaded, they cannot touch each other's data structures at all or things will go bad/crash, and there's no nice way to set it up to avoid this (my colleague who currently owns MSVCRT suggested a not-very-nice way to do it, but I don't think it's going to be reliable enough). Python's stable ABI helps, but does not solve this problem.
The file APIs are the worst culprits. The layout of FILE* objects can and do change between CRT versions, and file descriptors are simply indices into an array of these objects that is exposed through macros rather than function calls. As a result, you cannot mix either FILE pointers or file descriptors between CRTs. The only safe option is to build with the matching CRT, and for MSVCRT, this means with the matching compiler. It's unfortunate, and the responsible teams are well aware of the limitation, but it's history at this point, so we have no choice but to work with it.
Cheers,
Steve
Dear all,
I guess this is so obvious that someone must have suggested it before:
in list comprehensions you can currently exclude items based on the if
conditional, e.g.:
[n for n in range(1,1000) if n % 4 == 0]
Why not extend this filtering by allowing a while statement in addition to
if, as in:
[n for n in range(1,1000) while n < 400]
Trivial effect, I agree, in this example since you could achieve the same by
using range(1,400), but I hope you get the point.
This intuitively understandable extension would provide a big speed-up for
sorted lists where processing all the input is unnecessary.
Consider this:
some_names=["Adam", "Andrew", "Arthur", "Bob", "Caroline","Lancelot"] #
a sorted list of names
[n for n in some_names if n.startswith("A")]
# certainly gives a list of all names starting with A, but .
[n for n in some_names while n.startswith("A")]
# would have saved two comparisons
Best,
Wolfgang
On Fri, Nov 22, 2013 at 1:44 PM, Gregory P. Smith <greg(a)krypto.org> wrote:
> It'd be nice to formalize a way to get rid of the __name__ == '__main__'
> idiom as well in the long long run. Sure everyone's editor types that for
> them now but it's still a wart. Anyways, digressing... ;)
This has come up before and is the subject of several PEPS. [1][2]
The current idiom doesn't bother me too much as I try not to have
files that are both scripts and modules. However, Python doesn't make
the distinction all that clear nor does it do much to encourage people
to keep the two separate. I'd prefer improvements in both those
instead, but haven't had the time for any concrete proposal.
FWIW, aside from the idiom there are other complications that arise
from a module that also gets loaded in __main__ (run as a script).
See PEP 395 [3].
-erc
[1] http://www.python.org/dev/peps/pep-0299/
[2] http://www.python.org/dev/peps/pep-3122/
[3] http://www.python.org/dev/peps/pep-0395/ (sort of related)
Hello,
Coming back to python after a long time.
My present project is about (yet another) top-down matching / parsing lib. There
are 2 issues that, I guess, may be rather easily solved by simple string
methods. The core point is that any scanning / parsing process ends up, a the
lowest level, constantly comparing either single-char (rather single-code)
substrings or constant (literal) substrings of the source string. This is the
only operation which, when successful, actually advances in the source. Thus, it
is certainly worth having it efficient, or at the minimum not having it
needlessly inefficient. I suppose the same functionalities can be highly useful
in various other use cases of text processing.
Note again that I'm rediscovering Python (with some pleasure :-), thus may miss
known solutions -- but I asked on the tutor mailing list.
In both cases, I guess ordinary idiomatic Python code actually _creates_ a new
string object, as a substring of length 1 or more, which is otherwise useless;
for instance:
if s[i] == char:
# match ok -- object s[i] unneeded
if s[i:j] == substr:
# match ok -- object s[i:j] unneeded
What is actually needed is just to check for equality (or another check about a
code, see below).
The case of single-code checking appears when (1) a substring happens to hold a
single code (meaning it represents a simple or precomposed unicode char) (2)
when matching a char from a given set, range, or more complex class (eg in regex
[a-zA-Z0-9_-']). In all cases, what we want is tocheck the code: compare it to a
constant value, check whether it belongs to a set of value, or lies inside a
given range. We need the code --not a single-code string. Ideally, I'd like
expressions like:
c = s.code(i) # or s.ord(i) or s.ucode(i) [3]
# and then one of:
if c = code:
# match ok
if c in codes:
# match ok
if c >= code1 and c <= code2:
# match ok
The builtin function ord(char) does not do the job, since it only works for a
single-char string. We would again need to create a new string, with ord(s[i]).
The right solution apparently is a string method like code(self, i) giving the
code at an arbitrary index. I guess this is trivial.
I'm surprised it does not exist; maybe some may think this is a symptom there is
no strong need for it; instead, I guess people routinely use a typical Python
idiom without even noticing it creates a unneeded string object. [2] [3]
What do you think?
A second need is checking substring equality against constant substrings of
arbitrary sizes. This is similar to startswith & endswith, except at any code
index in the source string; a generalisation. In C implementation, it would
probably delegate to memcomp, with a start pointer set to p_source+i. On the
Python side, it may be a string method like sub_equals(self, substr, i). Choose
you preferred name ;-). [1] [4]
if s.sub_equals(substr, i):
# match ok
What do you think? (bis)
Thank you,
Denis
[1] I am unsure whether an end index is useful, actually I don't really
understand its usage for startswith & endswith neither.
[2] Actually, the compiler, if smart enough, may eliminate this object
construction and just check the code; does it? Anyway, I think it is not that
easy in the cases of ranges & sets.
[3] As a side-note, 'ord' is in my view a misnomer, since character codes are
not ordinals, with significant order, but nominals, plain numerical codes which
only need to be all distinct; they are kinds of id's. For unicode, I call them
'ucodes', an idea I stole somewhere. But I would be happy is the method is
called 'ord' anyway, since the term is established in the Python community.
[4] Would such a new method make startswith & endswith unneeded?
http://docs.python.org/2/library/string.html#template-strings
## Original Idea
stdlib lacks the most popular basic variable extension syntax
"{{ variable }}" that can be found in Django [1], Jinja2 [2] and
other templating engines [3].
## stdlib Analysis
string.Template syntax is ancient (dates back to Python 2.4
from 9 years ago). I haven't seen a template like this for a long time.
## Scope of Enhancement
st = 'Hello {{world}}.'
world = 'is not enough'
t = Template(string, style='brace')
t.render(locals())
## Links
1. https://docs.djangoproject.com/en/dev/topics/templates/#variables
2. http://jinja.pocoo.org/docs/templates/#variables
3. http://mustache.github.io/
## Feature Creeping
# Allow to override {{ }} symbols to make it more generic.
# `foo.bar` attribute lookup for 2D (nested) structures.
Questions is it has to be supported:
`foo.bar` in Django does dictionary lookup first, then attribute lookup
`foo.bar` in Jinja2 does attribute lookup first
I am not sure which is better. I definitely don't want some method or
property on a dict passed to render() method to hide dict value.
--
anatoly t.
Hello all,
I would like to propose refactoring the assertions out of
unittest.TestCase. As code speaks louder than words, you can see my
initial work at https://github.com/OddBloke/cpython.
The aim of the refactor is:
(a) to reduce the amount of repeated code in the assertions,
(b) to provide a clearer framework for third-party assertions to follow,
and
(c) to make it easier to split gnarly assertions in to an
easier-to-digest form.
My proposed implementation (as seen in the code above) is to move each
assertion in to its own class. There will be a shared superclass (called
Assert currently; see [0]) implementing the template pattern (look at
__call__ on line 69), meaning that each assertion only has to concern
itself with its unique aspects: what makes it fail, and how that
specific failure should be presented.
To maintain the current TestCase interface (all of the tests pass in my
branch), the existing assert* methods instantiate the Assert sub-classes
on each call with a context that captures self.longMessage,
self.failureException, self.maxDiff, and self._diffThreshold from the
TestCase instance.
Other potential aims include eventually deprecating the assertion
methods, providing a framework for custom assertions to hook in to, and
providing assertion functions (a la nose.tools) with a default context
set.
This proposal would help address #18054[1] as the new assertions could
just be implemented as separate classes (and not included in the
TestCase.assert* API); people who wanted to use them could just
instantiate them themselves.
I’d love some feedback on this proposal (and the implementation thus
far).
Cheers,
Dan (Odd_Bloke)
[0] https://github.com/OddBloke/cpython/blob/master/Lib/unittest/assertions/__i…
[1] http://bugs.python.org/issue18054
For a match object, m, m.group(0) is the semantics for accessing the entire
span of the match. For newcomers to regular expressions who are not
familiar with the concept of a 'group', the name group(0) is
counter-intuitive. A more natural-language-esque alias to group(0), perhaps
'matchSpan', could reduce the time novices spend from idea to working code.
Of course, this convenience would introduce a bit of complexity to the
codebase, so it may or may not be worth it to add an alias to group(0).
What do people think?
--
Alex Seewald
On 11/27/2013 11:06 AM, Ned Batchelder wrote:
>* While we're at it, how can it be that we haven't improved the __repr__ after all
*>* these years?
*>>* >>> m = re.search("[ab]", "xay")
*>* >>> m
*>* <_sre.SRE_Match object at 0x10a2ce9f0>
*>>* _sre? SRE_Match? huh? :)
*>>
* --Ned.*
Actually, it's fixed in Python 3.4, see http://bugs.python.org/issue17087.
Python 3.4.0b1 (default:9c7ab3e68243, Nov 26 2013, 19:42:06)
[GCC 4.2.1 20070831 patched [FreeBSD]] on freebsd8
Type "help", "copyright", "credits" or "license" for more information.
>>> import re
>>> re.search('aaa', 'aaab')
<_sre.SRE_Match object; span=(0, 3), match='aaa'>
>>>
