Re: Import without executing module
On Tue, 03 Feb 2009 20:08:34 -0800, Stephen Hansen wrote: There is no need to try to make sure something is executed/compiled only once in Python like you may want to do in C. Every module is only ever compiled once: if you import it ten times in ten different places only the first will compile and execute the code, the rest of the calls will return that original module again. nitpick If you import a module, it will be recompiled only if the module changes from the precompiled .pyc/.pyo. OTOH, if you run a module, it will be recompiled every time even if there is no change. /nitpick -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
Lie Ryan lie.1...@gmail.com wrote:
On Tue, 03 Feb 2009 20:08:34 -0800, Stephen Hansen wrote:
There is no need to try to make sure something is
executed/compiled only once in Python like you may want to do in C.
Every module is only ever compiled once: if you import it ten times in
ten different places only the first will compile and execute the code,
the rest of the calls will return that original module again.
nitpick
If you import a module, it will be recompiled only if the module changes
from the precompiled .pyc/.pyo. OTOH, if you run a module, it will be
recompiled every time even if there is no change.
/nitpick
As long as we are nitpicking, and trying to clarify things for a new
Python userlet me give it a try. By doing this I get to find out
if I really understand it as well as I think I do :)
If a file is run as a script ('python somefile.py', or shebang magic on
unix or the equivalent on other OSes), then the code is compiled/executed
each time that is done. If a module is imported (or something is imported
from a module), then the _first time_ during a given run of the python
interpreter that such an import is done, the timestamp of the .py file
(if it exists) is compared to the timestamp of the .pyc/.pyo file, and
if the .py is newer, the .py file is compiled to byte code, the resulting
byte code is written to the .pyc (or .pyo), and the byte code is executed.
If the .py file is older, then the saved byte code is loaded and executed.
It is the execution of the byte code that creates the module's namespace
(a collection of names pointing to objects). This namespace is registered
in sys.modules under the module's name. During the remainder of that
run of the python interpreter, any import statement that draws from
that module simply loads a pointer to the module's namespace (or to
objects referenced by the module's namespace if it is a 'import from')
into the local namespace.
--RDM
--
http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
On Feb 3, 1:37 pm, Ray rayky...@gmail.com wrote: I'll enclose the top-level commands with the if statement above...its just a minor change, but it seems unavoidable. Thanks again! Ray If you really don't want the file to be changed, you could (depends on the module) use the module as a subprocess. The ideal solution is for the module to have an if __name__ == '__main__': to determine whether it is being used as a module or a standalone program though. -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
Hi Lie, On Feb 3, 7:21 pm, Lie lie.1...@gmail.com wrote: On Feb 3, 1:37 pm, Ray rayky...@gmail.com wrote: I'll enclose the top-level commands with the if statement above...its just a minor change, but it seems unavoidable. Thanks again! Ray If you really don't want the file to be changed, you could (depends on the module) use the module as a subprocess. The ideal solution is for the module to have an if __name__ == '__main__': to determine whether it is being used as a module or a standalone program though. Thank you for this! I've done the if __name__ ... as you and others suggested. It isn't what I would have liked, but I'm ok with it...especially after finding out that top-level code has to be run while importing. As I'm new to python, I didn't know about that, but I now see there's no way around it. So, is inserting the above if statement common practice for python programmers? As a C programmer, it seems that people put a #ifndef XXX...#define XXX...[all of the code]...#endif almost as a habit. I wonder if its the same thing? Thank you! Ray -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
On Tue, Feb 3, 2009 So, is inserting the above if statement common practice for python programmers? As a C programmer, it seems that people put a #ifndef XXX...#define XXX...[all of the code]...#endif almost as a habit. I wonder if its the same thing? That statement is very common in Python. If a module is only ever going to be run as the main script its not needed but as you saw with this thread, sometimes you want to import what would be main into other places. More commonly I've found, is the use of that statement in other modules that are never -really- meant to be main but sometimes you want to run on their for testing or other purposes. For me, I use it all the time in my GUI modules that are never in the normal course of events run directly, but when developing them its useful to quickly start up a fake app / frame and load what I'm working on into a test display. That said, its use is fairly unique and idiomatic just as the 'main entry point' of a module. 75%(a number I'm pulling outta my arse!) of its uses are things like: if __name__ == __main__: main() Or whatever other minimal scaffolding is required to jumpstart the script's driver functions. Its unlikely you'll see anything like it elsewhere -- and it serves a very different purpose from what you describe seeing in C. There is no need to try to make sure something is executed/compiled only once in Python like you may want to do in C. Every module is only ever compiled once: if you import it ten times in ten different places only the first will compile and execute the code, the rest of the calls will return that original module again. You're only likely to see if statements around definitions and larger blocks of top-level code if someone is doing something... complicated. Or if they're doing a big procedural script that's not really ever meant to be re-used (especially one that grows overtime and takes a life of its own). It happens but its really rare, from my experience at least. --S -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
Hi Stephen, On Feb 4, 1:08 pm, Stephen Hansen apt.shan...@gmail.com wrote: On Tue, Feb 3, 2009 So, is inserting the above if statement common practice for python programmers? As a C programmer, it seems that people put a #ifndef XXX...#define XXX...[all of the code]...#endif almost as a habit. I wonder if its the same thing? That statement is very common in Python. If a module is only ever going to be run as the main script its not needed but as you saw with this thread, sometimes you want to import what would be main into other places. More commonly I've found, is the use of that statement in other modules that are never -really- meant to be main but sometimes you want to run on their for testing or other purposes. For ... Oh, I see...I never thought of module testing as one possible uses of it -- but yes, I see that now. Makes sense to test each module individually and (in C, at least) to write a separate main() driver in another file each time... That said, its use is fairly unique and idiomatic just as the 'main entry point' of a module. 75%(a number I'm pulling outta my arse!) of its uses are things like: if __name__ == __main__: main() Or whatever other minimal scaffolding is required to jumpstart the script's driver functions. Its unlikely you'll see anything like it elsewhere -- and it serves a very different purpose from what you describe seeing in C. There is no need to try to make sure something ... Ah, ok! I'm still trying to get use to how Python does things from languages such as C...old habits [from other languages] are harder to break. :) If the above if statement is fairly common, then I think I'll start using it just to get into the habit of it. Thank you very much for your detailed explanation! Ray -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
On Mon, Feb 2, 2009 at 1:58 PM, Stephen Hansen apt.shan...@gmail.comwrote: On Sun, Feb 1, 2009 at 11:43 PM, Taskinoor Hasan taskinoor.ha...@csebuet.org wrote: Can anyone explain what is the necessity of executing whole script when importing. Isn't it enough to just put the module name in the namespace and execute when some function is called? I'm not sure if I'm going to explain this right-- so bear with me. The code: def something(other): return other + 1 Doesn't exist until that def statement is executed. def, class and such are not magical and immediately register something somewhere. Its a statement that has to be executed to get a result, and that result is bound as an assignment to the name specified. Its not really all that different from: something = lambda other: other + 1 It can't pick through the top level instructions to determine what to execute if imported vs run, as every statement has the possibility of producing some binding of name to value and until you go and execute it you won't know. You may do something like: try: import blah except: blah = None You have to execute all of that to get a value for 'blah'. In Python, there's nothing really special about a function vs any other value. They are all objects that are assigned to a name in a given namespace. You can do dynamic things at the top level to result in different things being bound to the modules namespace based upon the statements and expressions evaluated. Does that make sense? If not someone else'll have to explain :) It make sense :-). So my reasoning..let A is imported in B, i.e. name A is put in B's namespace. When we call something like A.a then the interpreter first resolve A in B's namespace, then to get a, it need to look up A's namespace. And there is no way to populate A's namespace without executing A, and as all statements, including 'def', 'class' etc., are treated in the same way, whole script need to be executed. So whether a script can be imported as module or not is mainly dependent on how the script is written and largely on the intention of the coder. --Stephen -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
Hi Stephen and everyone, On Feb 2, 4:36 pm, Stephen Hansen apt.shan...@gmail.com wrote: Maybe he can wrap the things he dont need inside if __name__ == '__main__': check. -- http://mail.python.org/mailman/listinfo/python-list Yeah but he said he doesn't want to modify the file itself-- if he can modify the file this can all go away readily, yes. Thank you for your posts and your explanations; and yes, as you (Stephen) said here, I'd prefer not to modify it and use it as-is. Ideally, the original author could have separated the functions from the top-level code and I would just import the module with the functions. But that's whining since the hard work of writing the functions was already done and I'm very grateful for it; likewise, I'd like to use of the module unchanged. As there is no way around it, I'll look into ways of minimizing the number of changes and the if statement mentioned by Kottiyath would be my next step. Thank you all for the replies! Ray -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
On Feb 2, 11:51 pm, pyt...@bdurham.com wrote: If the output is coming from a print command, couldn't the OP temporarily redirect STDIO to a file to prevent the output from being displayed? He could, but that'd be a kludge on top of a stuff-up. He should put the script-only statements inside if __name__ == '__main__': and send the result back to the original author, who may in fact appreciate being enlightened :-) Cheers, John -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
If the output is coming from a print command, couldn't the OP temporarily redirect STDIO to a file to prevent the output from being displayed? Malcolm - Original message - From: Stephen Hansen apt.shan...@gmail.com To: Ray rayky...@gmail.com Cc: python-list@python.org Date: Sun, 1 Feb 2009 23:19:09 -0800 Subject: Re: Import without executing module On Sun, Feb 1, 2009 at 11:05 PM, Ray rayky...@gmail.com wrote: Basically, someone has created a python script and I would like to make use of his functions. I would prefer to not modify his file so what I would like to do is just write my script and import parts that are needed. i.e., I would like to separate my changes from his as best as I can. However, that other module has both functions (def's, which I would like to use) and top-level commands which I don't need and in fact, prints errors when I import it since it was meant to be run as a top-level module and not imported in. i.e., its expecting arguments to be supplied. Unfortunately, that's not possible, I believe. All the top level commands in a particular Python script are executed: that's how the functions get created. --S -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
Taskinoor Hasan wrote: [...] It make sense :-). So my reasoning..let A is imported in B, i.e. name A is put in B's namespace. When we call something like A.a then the interpreter first resolve A in B's namespace, then to get a, it need to look up A's namespace. And there is no way to populate A's namespace without executing A, and as all statements, including 'def', 'class' etc., are treated in the same way, whole script need to be executed. So whether a script can be imported as module or not is mainly dependent on how the script is written and largely on the intention of the coder. That's right! regards Steve -- Steve Holden+1 571 484 6266 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
Hi all, On Feb 3, 1:11 am, John Machin sjmac...@lexicon.net wrote: On Feb 2, 11:51 pm, pyt...@bdurham.com wrote: If the output is coming from a print command, couldn't the OP temporarily redirect STDIO to a file to prevent the output from being displayed? He could, but that'd be a kludge on top of a stuff-up. He should put the script-only statements inside if __name__ == '__main__': and send the result back to the original author, who may in fact appreciate being enlightened :-) Cheers, John Thanks John and Malcolm! In my case the first solution would not work. Sorry, that my simple test example was only a minimal one which did not reflect the true problem. The module that I do not want to edit not only runs on the command line and does some processing, but also expects arguments. So, when reading it through importing, it exits with an error (because no arguments were supplied). I'll enclose the top-level commands with the if statement above...its just a minor change, but it seems unavoidable. Thanks again! Ray -- http://mail.python.org/mailman/listinfo/python-list
Import without executing module
Hi all, I'm quite new to python as I've only just started to learn about it a few days ago... I am trying to do something and after reading about it, I'm still not sure whether or not it can be done. Basically, someone has created a python script and I would like to make use of his functions. I would prefer to not modify his file so what I would like to do is just write my script and import parts that are needed. i.e., I would like to separate my changes from his as best as I can. However, that other module has both functions (def's, which I would like to use) and top-level commands which I don't need and in fact, prints errors when I import it since it was meant to be run as a top-level module and not imported in. i.e., its expecting arguments to be supplied. For example, suppose I have a module test.py: - from test2 import foo foo() #import test2 #test2.foo() print test says hello - and test2.py: - def foo(): print foo is being executed print test2 says hello - and I would like to run python test.py without test2 says hello appearing. I figured import test2 would not work, so I thought that maybe from would only take the functions listed, but no. In the end, it is just importing all of test2 and then renaming it: foo = test2.foo . This isn't a very serious problem as I obviously have the source and can just copy the functions and attribute the original author (that's ok). But, it would be nice if I can make use of those functions without touching that file [test2.py in the above example]. Is this possible? Thank you! Ray -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
On Sun, Feb 1, 2009 at 11:05 PM, Ray rayky...@gmail.com wrote: Basically, someone has created a python script and I would like to make use of his functions. I would prefer to not modify his file so what I would like to do is just write my script and import parts that are needed. i.e., I would like to separate my changes from his as best as I can. However, that other module has both functions (def's, which I would like to use) and top-level commands which I don't need and in fact, prints errors when I import it since it was meant to be run as a top-level module and not imported in. i.e., its expecting arguments to be supplied. Unfortunately, that's not possible, I believe. All the top level commands in a particular Python script are executed: that's how the functions get created. --S -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
On Feb 2, 12:19 pm, Stephen Hansen apt.shan...@gmail.com wrote: On Sun, Feb 1, 2009 at 11:05 PM, Ray rayky...@gmail.com wrote: Basically, someone has created a python script and I would like to make use of his functions. I would prefer to not modify his file so what I would like to do is just write my script and import parts that are needed. i.e., I would like to separate my changes from his as best as I can. However, that other module has both functions (def's, which I would like to use) and top-level commands which I don't need and in fact, prints errors when I import it since it was meant to be run as a top-level module and not imported in. i.e., its expecting arguments to be supplied. Unfortunately, that's not possible, I believe. All the top level commands in a particular Python script are executed: that's how the functions get created. --S Maybe he can wrap the things he dont need inside if __name__ == '__main__': check. -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
Maybe he can wrap the things he dont need inside if __name__ == '__main__': check. -- http://mail.python.org/mailman/listinfo/python-list Yeah but he said he doesn't want to modify the file itself-- if he can modify the file this can all go away readily, yes. --S -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
Can anyone explain what is the necessity of executing whole script when importing. Isn't it enough to just put the module name in the namespace and execute when some function is called? On Mon, Feb 2, 2009 at 1:36 PM, Stephen Hansen apt.shan...@gmail.comwrote: Maybe he can wrap the things he dont need inside if __name__ == '__main__': check. -- http://mail.python.org/mailman/listinfo/python-list Yeah but he said he doesn't want to modify the file itself-- if he can modify the file this can all go away readily, yes. --S -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: Import without executing module
On Sun, Feb 1, 2009 at 11:43 PM, Taskinoor Hasan taskinoor.ha...@csebuet.org wrote: Can anyone explain what is the necessity of executing whole script when importing. Isn't it enough to just put the module name in the namespace and execute when some function is called? I'm not sure if I'm going to explain this right-- so bear with me. The code: def something(other): return other + 1 Doesn't exist until that def statement is executed. def, class and such are not magical and immediately register something somewhere. Its a statement that has to be executed to get a result, and that result is bound as an assignment to the name specified. Its not really all that different from: something = lambda other: other + 1 It can't pick through the top level instructions to determine what to execute if imported vs run, as every statement has the possibility of producing some binding of name to value and until you go and execute it you won't know. You may do something like: try: import blah except: blah = None You have to execute all of that to get a value for 'blah'. In Python, there's nothing really special about a function vs any other value. They are all objects that are assigned to a name in a given namespace. You can do dynamic things at the top level to result in different things being bound to the modules namespace based upon the statements and expressions evaluated. Does that make sense? If not someone else'll have to explain :) --Stephen -- http://mail.python.org/mailman/listinfo/python-list
