Thanks for the feedback!
On 19/03/2011 7:44 PM, Glenn Linderman wrote:
Not all of the ideas below are complementary to each other, some are
either or, to allow different thoughts to be inspired or different
directions to be taken.
Thanks for starting a PEP.
On 3/18/2011 11:02 PM, Mark Hammond wrote:
The launcher should be as simple as possible (but no simpler.)
The launcher could be simpler if it isn't used for launching interactive
interpreters as well as script references via Windows associations (more
about that after the next quote).
The launcher could be simpler if it just read through the file of its
first parameter until it finds a line starting with #@ (process as a
Windows version of Unix #!) or starting without a # (error case). This
avoids the need parse such lines.
I don't agree with that at all. The rules for the shebang parsing are
very simple - see the man-page I reference in the pep - the file must
start with the characters '#!' and the entire command must be < 127
chars. The only real "parsing" needed is to check if the specified
command starts with one of 2 fixed strings. I believe this is simpler
than parsing multiple lines of the file.
> Remember, the typical
Windows user is not likely to place a #! line in their scripts in the
first place, so teaching them what a Unix #! line is, and how the
parameter after it should be something that Windows doesn't even use,
and the launcher has to work hard to interpret, is not as simple as
possible.
I disagree with various aspects of that - the "typical Windows user" is
not going to add a shebang or variation of to their Python source files,
period. IMO, the kind of user who would is already somewhat likely to
know of the shebang convention, so it would not be foreign. Those
remaining who are not familiar with it can simply be pointed at existing
docs etc for the shebang line and their new knowledge now works on more
than Windows (and more than Python on non-Windows platforms)
The launcher could be simpler if the Python installer placed versioned
Python executables on the PATH. Unfortunately, historically it hasn't.
If it did, would, or the launcher installer would place them there for
pre-existing versions of Python, then the launcher could work by
launching the appropriate version of Python, expecting Windows to find
it on the PATH. The PEP doesn't address the level of internal complexity
of the launcher necessary to find where Python has been installed,
neither for CPython, nor for the alternate implementations to be supported.
The PEP intentionally doesn't, but the implementation does - it already
does exactly that for CPython. Other implementation may need a
different strategy, but we can cross that bridge when we come to it.
The launcher could be simpler if a directory \usr\bin were created under
Windows Program Files, placed on the PATH, and %ProgramFiles% prepended
to the Unix #! line, with the Python/Jython/Cython installers placing
appropriately versioned executables in that directory. Could even start
a trend for programs ported from Unix. One could even place an "env"
program there, for more simplicity.
Again, I disagree - I think in practice the code would be more complex,
and having Python assert it "owns" such directories and executables is a
can of worms we should avoid.
* When used to launch an interactive Python interpreter, the launcher
will support the first command-line argument optionally be a
version specifier in the form "-n[.n]" (where n is a single
integer) to nominate a specific version be used. For example,
while "py.exe" may locate and launch the latest Python 2.x
implementation installed, a command-line such as "py.exe -3" could
specify the latest Python 3.x implementation be launched, while
"py.exe -2.6" could specify Python 2.6 be located and launched.
If a Python 2.x implementation is desired to be launched with the
-3 flag, the command-line would need to be similar to "py.exe -2
-3" (or the specific version of Python could obviously be
launched manually without use of this launcher.)
I think that a python launcher that is "on the PATH" that could be used
to launch an interactive Python, should be different than one that is
used to launch XXXX.py[w] scripts.
I believe that if you know the Python you want is already on the PATH,
you should just use 'python.exe' instead of 'py.exe'. I don't see any
reason to use this launcher for interactive Python sessions where this
is the case.
1) python should be invoked interactively by typing "python" or
"pythonX[.Y]" at the CMD prompt, not "py". This can be done without a
launcher, if appropriate versioned pythons are placed on the PATH. The
launcher is really and only needed for handling XXXX.py[w] scripts,
which, in the Windows way of thinking, can only be associated with one
specific, system-wide configured version of Python (presently, the
latest one wins). The script itself is not examined to modify such an
association. The Unix !# line provides such modification on Unix.
OK, I think we found something we can agree on :) If the python.exe you
want is on your path, just ignore this launcher for anything other than
file associations.
2) If the launcher provides command line options for the "benefit" of
launching interactive Python interpreters, those command line options
can have data puns with script names, or can conflict with actual Python
options. I believe Python 2 already has a -3 option, for example. And
Windows users are not trained that "-" introduces an option syntax, but
rather "/". Most _programmer_ users would probably be aware of "-" as an
option syntax, but Python is used as a language for non-programmers in
some circles, and few Windows non-programmers understand "/" much less
"-" and not even command lines very well. So not using a launcher for
launching interactive Python sidesteps all that: Python itself is
introduced and taught, and no need to teach about (or even have)
launcher options that could possibly conflict and confuse, in addition
to Python options that may conflict with script names already. (I have
seen lots of Windows users use leading punctuation characters on
filenames to affect sort order and grouping of files in My Documents,
not knowing they can create subdirectories/subfolders, or not wanting to
bother with them, since all their applications default to opening things
from My Documents.)
I'm not 100% sure of the points you are trying to make above, but the
gist of it seems to be the same as (1) - use python.exe directly if you
prefer - in which case I agree. Obviously if people choose to use the
new launcher interactively they are doing so because they see some
benefit and would be willing to understand how it works. So I don't see
any problems here as I'm not advocating it would replace "python.exe" in
interactive scenarios except where the user actively chooses to.
3) Unix !# lines can have embedded options after the program name on the
line. Such options would be another source of potential conflict with
launcher options, if the launcher has options for use with launching
interactive interpreters.
Item 3 is also an issue for the PEP even apart from its use as an
interactive Python launcher; since options may exist on the Unix #!
line, a discussion of how and if they are handled by the launcher should
be included in the PEP.
I believe a reference to the execve man-page and the note in the PEP
that we will support the description there, including limitations, is
enough - but I'm happy to change this if people agree it is
underspecified or confusing.
* Environment varialbes will be used to override the semantics for
determining exactly what version of Python will be used. For
example, while a shebang line of "/usr/bin/python2" will
automatically locate a Python 2.x implementation, an environment
variable can override exactly which Python 2.x implementation will
be chosen. Similarly for "/usr/bin/python" etc.
Clarify if environment variables can be used to override semantics for
shebang lines of the form "/usr/bin/python2.x".
On re-reading the PEP, I notice I deleted something which is important
on the mistaken belief the execve man page would make it clear. Only
the first command-line argument will be checked for a shebang line. If
the first argument is an option (ie, starts with a '-'), nothing is
examined for a shebang line. This is what the reference implementation
does and I'll be sure to update the PEP to reflect this.
The PEP does state the optional -N.N argument must be the first
argument. Therefore, the presence of the -N.N argument will avoid any
shebang processing at all so can't impact the version selected via the
shebang line. This makes sense to me as someone explicitly executing
"py.exe -3 foo.py" is explicitly overriding the version they want to run
the script with.
If alternate implementations are to be supported, additional virtual
commands will be required, not just these two. Each one adds complexity
to the launcher.
This is true - each implementation would need custom code to sniff out
the requested implementation version. I don't think this is a burden
though and can be worked around by using fully-qualified executable
names in the shebang line.
Non-virtual shebang lines should be discouraged as they make the
script specific to a specific Windows installation. However, they
are supported for maximum flexibility.
This is a false statement.
I believe it is a true statement - needing words like "likely" and
"most" to dispute it just demonstrates that IMO. Note that I don't
dispute your statements, but instead believe that "most" simply isn't
good enough - I want "always".
Unfortunately, it is clear we disagree on some fundamental points and I
can't see a way to change my PEP that would keep both you and I happy.
Therefore, the only reasonable resolution would be for you to draft a
competing PEP and reference implementation which python-dev can
informally vote on.
Cheers,
Mark
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