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package imports, sys.path and os.chdir()

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tismer at stackless

Apr 26, 2012, 2:30 PM

Post #1 of 18 (342 views)
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package imports, sys.path and os.chdir()

Howdy,

I have a small problem/observation with imports.

I have several packages to import, which works all fine, as long
as the packages are imported from directories found on the installed
site-packages, via .pth etc.

The only problem is the automatically prepended empty string in sys.path.
Depending from where I start my application, the values stored
in package.__file__ and package.__path__ are absolute or relative
paths.

So, if my pwd is the directory that contains my top-level modules,
even though sys.path contains correct absolute entries for that, in this
case the '' entry wins.

Assume this:

<- cwd is here
moda
modb

>>> import moda

Some code happens to chdir away, and later some code does

>>> from moda import modb

Since the __path__ entry is now a relative path, this second import fails.

Although it is no recommended practice to leave a changed chdir(), I
don't see why this is so. When a module is imported, would it not be
better to always make __file__ and __path__ absolute?

I see the module path, hidden by the '' entry not as a feature but
an undesired side-effect.

No big deal and easy to work around, I just would like to understand why.

cheers -- chris

--
Christian Tismer :^)<mailto:tismer [at] stackless>
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ncoghlan at gmail

Apr 26, 2012, 5:39 PM

Post #2 of 18 (329 views)
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Re: package imports, sys.path and os.chdir() [In reply to]

On Fri, Apr 27, 2012 at 7:30 AM, Christian Tismer <tismer [at] stackless> wrote:
> No big deal and easy to work around, I just would like to understand why.

I don't like it either and want to change it, but I'm also not going
to mess with it until the importlib bootstrapping is fully integrated
and stable.

For the moment, there's a workaround in runpy to ensure at least
__main__.__file__ is always absolute (even when using the -m switch).
Longer term, I'd like to see __file__ and __path__ entries to be
guaranteed to be *always* absolutely, even when they're imported
relative to the current working directory.

Cheers,
Nick.

--
Nick Coghlan | ncoghlan [at] gmail | Brisbane, Australia
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tismer at stackless

Apr 27, 2012, 7:39 AM

Post #3 of 18 (324 views)
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Re: package imports, sys.path and os.chdir() [In reply to]

On 27.04.12 02:39, Nick Coghlan wrote:
> On Fri, Apr 27, 2012 at 7:30 AM, Christian Tismer<tismer [at] stackless> wrote:
>> No big deal and easy to work around, I just would like to understand why.
> I don't like it either and want to change it, but I'm also not going
> to mess with it until the importlib bootstrapping is fully integrated
> and stable.
>
> For the moment, there's a workaround in runpy to ensure at least
> __main__.__file__ is always absolute (even when using the -m switch).
> Longer term, I'd like to see __file__ and __path__ entries to be
> guaranteed to be *always* absolutely, even when they're imported
> relative to the current working directory.
>

Is there a recommendable way to fix this? I would like to tell people
what to do to make imports reliable. Either I put something into
the toplevel __init__ code, or I hack something into .pth or sitecustomize,
and then forget about this.

But I fear hacking __init__ is the only safe way that works without
a special python setup, which makes the whole reasoning rather
useless, because I can _not_ forget about this.... waah ;-)

cheers - chris

--
Christian Tismer :^)<mailto:tismer [at] stackless>
tismerysoft GmbH : Have a break! Take a ride on Python's
Karl-Liebknecht-Str. 121 : *Starship* http://starship.python.net/
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brett at python

Apr 27, 2012, 1:00 PM

Post #4 of 18 (322 views)
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Re: package imports, sys.path and os.chdir() [In reply to]

On Fri, Apr 27, 2012 at 10:39, Christian Tismer <tismer [at] stackless>wrote:

> On 27.04.12 02:39, Nick Coghlan wrote:
>
>> On Fri, Apr 27, 2012 at 7:30 AM, Christian Tismer<tismer [at] stackless>
>> wrote:
>>
>>> No big deal and easy to work around, I just would like to understand why.
>>>
>> I don't like it either and want to change it, but I'm also not going
>> to mess with it until the importlib bootstrapping is fully integrated
>> and stable.
>>
>> For the moment, there's a workaround in runpy to ensure at least
>> __main__.__file__ is always absolute (even when using the -m switch).
>> Longer term, I'd like to see __file__ and __path__ entries to be
>> guaranteed to be *always* absolutely, even when they're imported
>> relative to the current working directory.
>>
>>
> Is there a recommendable way to fix this? I would like to tell people
> what to do to make imports reliable. Either I put something into
> the toplevel __init__ code, or I hack something into .pth or sitecustomize,
> and then forget about this.
>
>
No, there isn't.

> But I fear hacking __init__ is the only safe way that works without
> a special python setup, which makes the whole reasoning rather
> useless, because I can _not_ forget about this.... waah ;-)
>

Yeah, to guarantee the semantics you are after you have to grab that ''
entry in sys.path as early as possible and substitute it with the cwd so
that its initial value propagates through the interpreter. Importlib is
already having to jump through some hoops to treat it as '.' and even that
doesn't get you what you want since that will change when the cwd is moved.

I'm personally in favour of changing the insertion of '' to sys.path to
inserting the cwd when the interpreter is launched.

v+python at g

Apr 27, 2012, 1:37 PM

Post #5 of 18 (323 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On 4/27/2012 1:00 PM, Brett Cannon wrote:
> I'm personally in favour of changing the insertion of '' to sys.path
> to inserting the cwd when the interpreter is launched.
+1

tismer at stackless

Apr 27, 2012, 2:21 PM

Post #6 of 18 (323 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On 27.04.12 22:00, Brett Cannon wrote:
>
>
> On Fri, Apr 27, 2012 at 10:39, Christian Tismer <tismer [at] stackless
> <mailto:tismer [at] stackless>> wrote:
>
> On 27.04.12 02:39, Nick Coghlan wrote:
>
> On Fri, Apr 27, 2012 at 7:30 AM, Christian
> Tismer<tismer [at] stackless <mailto:tismer [at] stackless>> wrote:
>
> No big deal and easy to work around, I just would like to
> understand why.
>
> I don't like it either and want to change it, but I'm also not
> going
> to mess with it until the importlib bootstrapping is fully
> integrated
> and stable.
>
> For the moment, there's a workaround in runpy to ensure at least
> __main__.__file__ is always absolute (even when using the -m
> switch).
> Longer term, I'd like to see __file__ and __path__ entries to be
> guaranteed to be *always* absolutely, even when they're imported
> relative to the current working directory.
>
>
> Is there a recommendable way to fix this? I would like to tell people
> what to do to make imports reliable. Either I put something into
> the toplevel __init__ code, or I hack something into .pth or
> sitecustomize,
> and then forget about this.
>
>
> No, there isn't.
>
> But I fear hacking __init__ is the only safe way that works without
> a special python setup, which makes the whole reasoning rather
> useless, because I can _not_ forget about this.... waah ;-)
>
>
> Yeah, to guarantee the semantics you are after you have to grab that
> '' entry in sys.path as early as possible and substitute it with the
> cwd so that its initial value propagates through the interpreter.
> Importlib is already having to jump through some hoops to treat it as
> '.' and even that doesn't get you what you want since that will change
> when the cwd is moved.
>
> I'm personally in favour of changing the insertion of '' to sys.path
> to inserting the cwd when the interpreter is launched.

Thanks Brett, that sounds pretty reasonable. '' always was too implicit
for me.

cheers - chris

--
Christian Tismer :^)<mailto:tismer [at] stackless>
tismerysoft GmbH : Have a break! Take a ride on Python's
Karl-Liebknecht-Str. 121 : *Starship* http://starship.python.net/
14482 Potsdam : PGP key -> http://pgp.uni-mainz.de
work +49 173 24 18 776 mobile +49 173 24 18 776 fax n.a.
PGP 0x57F3BF04 9064 F4E1 D754 C2FF 1619 305B C09C 5A3B 57F3 BF04
whom do you want to sponsor today? http://www.stackless.com/

ncoghlan at gmail

Apr 28, 2012, 1:08 AM

Post #7 of 18 (318 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On Sat, Apr 28, 2012 at 6:00 AM, Brett Cannon <brett [at] python> wrote:
> I'm personally in favour of changing the insertion of '' to sys.path to
> inserting the cwd when the interpreter is launched.

I'm not, because it breaks importing from the interactive prompt if
you change directory after starting the session.

The existing workaround for applications is pretty trivial:

# Somewhere in your initialisation code
for i, entry in enumerate(sys.path):
sys.path[i] = os.path.abspath(i)

The fix for the import system is similarly trivial: call
os.path.abspath when calculating __file__ (just as runpy now does and
the import emulation in pkgutil always has).

Cheers,
Nick.

--
Nick Coghlan | ncoghlan [at] gmail | Brisbane, Australia
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benjamin at python

Apr 28, 2012, 6:35 AM

Post #8 of 18 (315 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

2012/4/28 Nick Coghlan <ncoghlan [at] gmail>:
> On Sat, Apr 28, 2012 at 6:00 AM, Brett Cannon <brett [at] python> wrote:
>> I'm personally in favour of changing the insertion of '' to sys.path to
>> inserting the cwd when the interpreter is launched.
>
> I'm not, because it breaks importing from the interactive prompt if
> you change directory after starting the session.
>
> The existing workaround for applications is pretty trivial:
>
> Â # Somewhere in your initialisation code
> Â for i, entry in enumerate(sys.path):
> Â Â Â sys.path[i] = os.path.abspath(i)
>
> The fix for the import system is similarly trivial: call
> os.path.abspath when calculating __file__ (just as runpy now does and
> the import emulation in pkgutil always has).

I thought __file__ was required to be absolute in Python 3.

--
Regards,
Benjamin
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rdmurray at bitdance

Apr 28, 2012, 9:16 AM

Post #9 of 18 (313 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On Sat, 28 Apr 2012 18:08:08 +1000, Nick Coghlan <ncoghlan [at] gmail> wrote:
> On Sat, Apr 28, 2012 at 6:00 AM, Brett Cannon <brett [at] python> wrote:
> > I'm personally in favour of changing the insertion of '' to sys.path to
> > inserting the cwd when the interpreter is launched.
>
> I'm not, because it breaks importing from the interactive prompt if
> you change directory after starting the session.

Heh. I've never thought of doing that. I would not have expected it
to work (change directory from the interactive prompt and be able to
import something located in the new cwd). I don't know why I wouldn't
have expected it to work, I just didn't.

That said, could this insertion of '' only happen when the interactive
prompt is actually posted, and otherwise use cwd?

--David
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brett at python

Apr 28, 2012, 12:16 PM

Post #10 of 18 (313 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On Sat, Apr 28, 2012 at 04:08, Nick Coghlan <ncoghlan [at] gmail> wrote:

> On Sat, Apr 28, 2012 at 6:00 AM, Brett Cannon <brett [at] python> wrote:
> > I'm personally in favour of changing the insertion of '' to sys.path to
> > inserting the cwd when the interpreter is launched.
>
> I'm not, because it breaks importing from the interactive prompt if
> you change directory after starting the session.
>
>
Who does that? I mean what possible need do you have to start the
interpreter in one directory, but then need to chdir somewhere else where
you are doing your actual importing from, and in a way where you can't
simply attach the directory you want to use into sys.path?

> The existing workaround for applications is pretty trivial:
>
> # Somewhere in your initialisation code
> for i, entry in enumerate(sys.path):
> sys.path[i] = os.path.abspath(i)
>
> The fix for the import system is similarly trivial: call
> os.path.abspath when calculating __file__ (just as runpy now does and
> the import emulation in pkgutil always has).
>

You say trivial, I say a pain as that means porting over os.path.abspath()
into importlib._bootstrap that works for all platforms.

-Brett

>
> Cheers,
> Nick.
>
> --
> Nick Coghlan | ncoghlan [at] gmail | Brisbane, Australia
>

brett at python

Apr 28, 2012, 12:17 PM

Post #11 of 18 (316 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On Sat, Apr 28, 2012 at 09:35, Benjamin Peterson <benjamin [at] python>wrote:

> 2012/4/28 Nick Coghlan <ncoghlan [at] gmail>:
> > On Sat, Apr 28, 2012 at 6:00 AM, Brett Cannon <brett [at] python> wrote:
> >> I'm personally in favour of changing the insertion of '' to sys.path to
> >> inserting the cwd when the interpreter is launched.
> >
> > I'm not, because it breaks importing from the interactive prompt if
> > you change directory after starting the session.
> >
> > The existing workaround for applications is pretty trivial:
> >
> > # Somewhere in your initialisation code
> > for i, entry in enumerate(sys.path):
> > sys.path[i] = os.path.abspath(i)
> >
> > The fix for the import system is similarly trivial: call
> > os.path.abspath when calculating __file__ (just as runpy now does and
> > the import emulation in pkgutil always has).
>
> I thought __file__ was required to be absolute in Python 3.
>

Not that I'm specifically aware of. Since site makes all entries in
sys.path absolute it is really only an issue if you launch without site or
the '' entry in sys.path.

-Brett

>
>
>
> --
> Regards,
> Benjamin
>

brett at python

Apr 28, 2012, 12:20 PM

Post #12 of 18 (310 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On Sat, Apr 28, 2012 at 12:16, R. David Murray <rdmurray [at] bitdance>wrote:

> On Sat, 28 Apr 2012 18:08:08 +1000, Nick Coghlan <ncoghlan [at] gmail>
> wrote:
> > On Sat, Apr 28, 2012 at 6:00 AM, Brett Cannon <brett [at] python> wrote:
> > > I'm personally in favour of changing the insertion of '' to sys.path to
> > > inserting the cwd when the interpreter is launched.
> >
> > I'm not, because it breaks importing from the interactive prompt if
> > you change directory after starting the session.
>
> Heh. I've never thought of doing that. I would not have expected it
> to work (change directory from the interactive prompt and be able to
> import something located in the new cwd). I don't know why I wouldn't
> have expected it to work, I just didn't.
>
> That said, could this insertion of '' only happen when the interactive
> prompt is actually posted, and otherwise use cwd?

If the decision to keep this entry around stands, can we consider changing
it to '.' instead of the empty string? It mucks up stuff if you are not
careful (e.g. ``os.listdir('')`` or ``"/".join(['', 'filename.py'])``).

tjreedy at udel

Apr 28, 2012, 7:02 PM

Post #13 of 18 (307 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On 4/28/2012 3:16 PM, Brett Cannon wrote:

> Who does that? I mean what possible need do you have to start the
> interpreter in one directory, but then need to chdir somewhere else
> where you are doing your actual importing from, and in a way where you
> can't simply attach the directory you want to use into sys.path?

Idle, at least on Windows, when started from the installed icon, starts
in the directory of the associated pythonw.exe. There is no choice. And
that is a bad place to put user files for import. So anyone using Idle
and importing user files does just what you think is strange. Windows
ain't *nix. If one opens a file in another directory*, that becomes the
new current directory and imports from that directory work. I would not
want that to change. I presume that changing '' to '.' would not change
that.

*and the easiest way to do *that* is from the 'recent files' list. I
almost never type a path on Windows.

--
Terry Jan Reedy

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steve at pearwood

Apr 28, 2012, 8:20 PM

Post #14 of 18 (308 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

Brett Cannon wrote:
> On Sat, Apr 28, 2012 at 04:08, Nick Coghlan <ncoghlan [at] gmail> wrote:
>
>> On Sat, Apr 28, 2012 at 6:00 AM, Brett Cannon <brett [at] python> wrote:
>>> I'm personally in favour of changing the insertion of '' to sys.path to
>>> inserting the cwd when the interpreter is launched.
>> I'm not, because it breaks importing from the interactive prompt if
>> you change directory after starting the session.
>>
>>
> Who does that?

Me.

You're asking this as if it were a bizarre and disturbing thing to do. It's
not as if changing directory is an unsupported hack.

When I use the Python interactive interpreter for interactive exploration or
testing, sometimes I discover I'm in the wrong directory. If I've just started
a fresh session, I'll probably just exit back to the shell, cd, then start
Python again. But if there's significant history in the current session, I'll
just change directories and continue on.

> I mean what possible need do you have to start the
> interpreter in one directory, but then need to chdir somewhere else where
> you are doing your actual importing from, and in a way where you can't
> simply attach the directory you want to use into sys.path?

Of course I could manipulate sys.path. But chances are that I still have to
change directory anyway, so that reading and writing data files go where I
want without having to specify absolute paths.

--
Steven

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pje at telecommunity

Apr 28, 2012, 8:41 PM

Post #15 of 18 (311 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On Sat, Apr 28, 2012 at 12:16 PM, R. David Murray <rdmurray [at] bitdance>wrote:

> That said, could this insertion of '' only happen when the interactive
> prompt is actually posted, and otherwise use cwd?
>

That's already the case. Actually, sys.path[0] is *always* the absolute
path of the script directory -- regardless of whether you invoked the
script by a relative path or an absolute one, and regardless of whether
you're importing 'site' -- at least on Linux and Cygwin and WIndows, for
all Python versions I've used regularly, and 3.2 besides.

It isn't the value of cwd unless you happen to run a script from the same
directory as the script itself. But even then, it's absolute, and not an
empty string: the empty string is only present for interactive sessions.

ncoghlan at gmail

Apr 28, 2012, 10:05 PM

Post #16 of 18 (312 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On Sun, Apr 29, 2012 at 1:41 PM, PJ Eby <pje [at] telecommunity> wrote:
> That's already the case. Actually, sys.path[0] is *always* the absolute
> path of the script directory -- regardless of whether you invoked the script
> by a relative path or an absolute one, and regardless of whether you're
> importing 'site' -- at least on Linux and Cygwin and WIndows, for all Python
> versions I've used regularly, and 3.2 besides.

"-c" and "-m" also insert the empty string as sys.path[0] in order to
find local files. They could just as easily insert the full cwd
explicitly though, and, in fact, they arguably should. (I say
arguably, because changing this *would* be a backwards incompatible
change - there's no such issue with requiring __file__ to be
absolute).

If we fixed that, then you could only get relative filenames from the
interactive prompt.

There's another way we can go with this, though: something I'm working
on at the moment is having usage of the frozen importlib be
*temporary*, switching to the full Python source version as soon as
possible (i.e. as soon as the frozen version is able to retrieve the
full version from disk).

There's a trick that becomes possible if we go down that path: we can
have some elements of importlib._bootstrap that *don't run* during the
initial bootstrapping phase.

Specifically, we can have module level code that looks like this:

if __name__.startswith("importlib."):
# Import system has been bootstrapped with the frozen version,
we now have full stdlib access
# and other parts of the interpreter have also been fully initialised
from os.path import abspath as _abspath
_debug_msg = print
else:
# Running from the frozen copy, there's things we can't do yet
because the interpreter is not fully configured
def _abspath(entry):
# During the bootstrap process, we let relative paths
slide. It will only happen if someone shadows the stdlib in their
# current directory.
return entry
def _debug_msg(*args, **kwds):
# Standard streams are not initialised yet
pass

Cheers,
Nick.

--
Nick Coghlan | ncoghlan [at] gmail | Brisbane, Australia
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tismer at stackless

Apr 29, 2012, 4:14 AM

Post #17 of 18 (302 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On 28.04.12 21:16, Brett Cannon wrote:
>
>
> On Sat, Apr 28, 2012 at 04:08, Nick Coghlan <ncoghlan [at] gmail
> <mailto:ncoghlan [at] gmail>> wrote:
>
> On Sat, Apr 28, 2012 at 6:00 AM, Brett Cannon <brett [at] python
> <mailto:brett [at] python>> wrote:
> > I'm personally in favour of changing the insertion of '' to
> sys.path to
> > inserting the cwd when the interpreter is launched.
>
> I'm not, because it breaks importing from the interactive prompt if
> you change directory after starting the session.
>
>
> Who does that? I mean what possible need do you have to start the
> interpreter in one directory, but then need to chdir somewhere else
> where you are doing your actual importing from, and in a way where you
> can't simply attach the directory you want to use into sys.path?
>

Well, it depends on which hat I'm wearing.

Scenario 1:
I am designing a big application. This application shall run without
problems,
with disambiguated imports, and by no means should hit anything that is not
meant to be imported.
In this case, I need to remove '' from sys.path and replace it with an
absolute entry.

Update: I see this works already unless "-c" and "-m" are present (hum).

Scenario 2:
I am playing with the application, want to try several modules, or even
several versions
of modules. I do use os.chdir() to get into a certain context, try
imports, remove them
again, chdir() to a different directory with a slightly changed module,
et cetera.
In this case, I need '' (or as has been mentioned '.') to have
flexibility for testing,
debugging and exploration.

These scenarios are both perfectly valid for their use case, but they
have pretty
different implication for imports, and especially for sys.path.

So the real question I was after was "can os.chdir() be freely used?"

It would be great to get "yes" or "no", but the answer is right now "it
depends".

cheers - chris

--
Christian Tismer :^)<mailto:tismer [at] stackless>
tismerysoft GmbH : Have a break! Take a ride on Python's
Karl-Liebknecht-Str. 121 : *Starship* http://starship.python.net/
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tismer at stackless

Apr 29, 2012, 6:37 AM

Post #18 of 18 (286 views)
Permalink

Re: package imports, sys.path and os.chdir() [In reply to]

On 29.04.12 07:05, Nick Coghlan wrote:
> On Sun, Apr 29, 2012 at 1:41 PM, PJ Eby<pje [at] telecommunity> wrote:
>> That's already the case. Actually, sys.path[0] is *always* the absolute
>> path of the script directory -- regardless of whether you invoked the script
>> by a relative path or an absolute one, and regardless of whether you're
>> importing 'site' -- at least on Linux and Cygwin and WIndows, for all Python
>> versions I've used regularly, and 3.2 besides.
> "-c" and "-m" also insert the empty string as sys.path[0] in order to
> find local files. They could just as easily insert the full cwd
> explicitly though, and, in fact, they arguably should. (I say
> arguably, because changing this *would* be a backwards incompatible
> change - there's no such issue with requiring __file__ to be
> absolute).

As a note: I tried to find out where and when the empty string actually
got inserted into sys.path. Not very easy, had to run the C debugger
to understand that:

It happens in sysmodule.c

PyMain
PySys_SetArgv(argc-_PyOS_optind, argv+_PyOS_optind);

that calls

PySys_SetArgvEx(int argc, char **argv, int updatepath)

and the logic weather to use the empty string or a full path etc.
is deeply hidden in a C function as a side effect. Brrrrrr!

It would be much cleaner and easier if that stuff would be ignored
today and called a Python implementation, instead.

Is that in the plans to get rid of C for such stuff? I hope so :-)

cheers -- Chris

--
Christian Tismer :^)<mailto:tismer [at] stackless>
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