I am a complete newb to python, hence a silly question.
As i understand, upon first execution of a *.py program, byte code is created into *.pyc and used until a change in *.py file.
Where might this *.pyc bytecode be found in a project?
I would think bin, but nothing is there
A *.pyc file is created for imported modules, and they are placed in the same directory containing the .py file. However... no .pyc file is created for the main script for your program. In other words... if you call "python myscript.py" on the command line, there will be no .pyc file for myscript.py.
This is how Python 2.x behaves. However, in Python 3.x the .pyc files are saved in a __pycache__ directory. See David Glick's answer for details.
[edit: Added note about Python 3.x.]
In Python < 3.2, the .pyc files are placed in the same directory as the .py file.
In Python 3.2, the compiled files are placed in a __pycache__ subdirectory, and are named differently depending on which Python interpreter created them. (This can be useful to people importing the same Python modules from multiple versions of Python.) See http://docs.python.org/dev/whatsnew/3.2.html#pep-3147-pyc-repository-directories for more information.
They are always created in whatever directory contains your .py files. Also, they are created for imported modules, not files that you directly run.
If you want to create .pyc files, boot up a Python interpreter and just import the modules of your choosing.
The .pyc files are normally created in the same directory as the files that they correspond to. E.g., both my_file.py and my_file.pyc should be in the same location.
Related
This answer tells me that a .pyc file gets created when a .py file is run, which I understand saves loading time when re-run. Which makes me wonder what the point of the .py file is after the .pyc is created.
When backing up my code, or sharing it, I don't want to include redundant or extraneous files. Which filetype should I focus on?
Side question: I have one script that calls another. After running them, the called script got a .pyc file written, but the master script that does the calling did not. Why would that be?
Python .pyc files are generated when a module is imported, not when a top level script is run. I'm not sure what you mean by calling, but if you ran your master script from the command line and it imported the other script, then only the imported one gets a .pyc.
As for distributing .pyc files, they are minor version sensitive. If you bundle your own python or distribute multiple python-version sensitive files, then maybe. But best practice is to distribute the .py files.
Python's script and module rules seem a bit odd until you consider its installation model. A common installation model is that executables are installed somewhere on the system's PATH and shared libraries are installed somewhere in a library path.
Python's setup.py does the same thing. Top level scripts go on the PATH but modules and packages go in an library path. For instance on my system, pdb3 (a top level script) is at /usr/bin/pdb3 and os (an imported module) is at /usr/lib/python3.4/os.py. Suppose python compiled pdb3 to pdb3.pyc. Well, I'd still call pdb3 and the .pyc is useless. So why clutter the path?
Its common for installs to run as root or administrator so you have write access on those paths. But you wouldn't have write access to them later as a regular user. You can have setup.py generate .pyc files during install. You get the right .pyc files for whatever python you happen to have, and since you are running as root/admin during install you still have acess to the directories. Trying to build .pyc files later is a problem because a regular user doesn't have access to the directories.
So, best practice is to distribute .py files and have setup.py build the .pyc during install.
If you simply want to run your Python script, all you really need is .pyc which is the bytecode generated from your source code. See here for details on running a .pyc file. I will warn that some of the detials are bit twisty.
However I recommend including your source code and leaving out your .pyc files as they are generated automatically by the Python Interpreter. Besides, if you, or another person would want to revise/revisit your source code at a later point, you would need the .py files. Furthermore, it is usually best practice to just include your source code.
I have a Python directory with a number of .py files. I recently compiled them into .pyc files using python -m compileall. I have now changed some of the source files and would like to recompile, writing over the old .pyc files.
Is there a quick way to do this from the command line without having to manually delete all existing .pyc files?
You just have to run python -m compileall again. It will overwrite older .pyc files. You can pass it the -f switch to force rebuild even if timestamps are up to date (as per the documentation).
When the source code has changed, new .pyc files are automatically created when you run the program again. Therefore I wouldn't worry about compiling, but focus your attention on the code itself.. :)
I understand that ".pyc" files are compiled versions of the plain-text ".py" files, created at runtime to make programs run faster. However I have observed a few things:
Upon modification of "py" files, program behavior changes. This indicates that the "py" files are compiled or at least go though some sort of hashing process or compare time stamps in order to tell whether or not they should be re-compiled.
Upon deleting all ".pyc" files (rm *.pyc) sometimes program behavior will change. Which would indicate that they are not being compiled on update of ".py"s.
Questions:
How do they decide when to be compiled?
Is there a way to ensure that they have stricter checking during development?
The .pyc files are created (and possibly overwritten) only when that python file is imported by some other script. If the import is called, Python checks to see if the .pyc file's internal timestamp is not older than the corresponding .py file. If it is, it loads the .pyc; if it isn't or if the .pyc does not yet exist, Python compiles the .py file into a .pyc and loads it.
What do you mean by "stricter checking"?
.pyc files generated whenever the corresponding code elements are imported, and updated if the corresponding code files have been updated. If the .pyc files are deleted, they will be automatically regenerated. However, they are not automatically deleted when the corresponding code files are deleted.
This can cause some really fun bugs during file-level refactors.
First of all, you can end up pushing code that only works on your machine and on no one else's. If you have dangling references to files you deleted, these will still work locally if you don't manually delete the relevant .pyc files because .pyc files can be used in imports. This is compounded with the fact that a properly configured version control system will only push .py files to the central repository, not .pyc files, meaning that your code can pass the "import test" (does everything import okay) just fine and not work on anyone else's computer.
Second, you can have some pretty terrible bugs if you turn packages into modules. When you convert a package (a folder with an __init__.py file) into a module (a .py file), the .pyc files that once represented that package remain. In particular, the __init__.pyc remains. So, if you have the package foo with some code that doesn't matter, then later delete that package and create a file foo.py with some function def bar(): pass and run:
from foo import bar
you get:
ImportError: cannot import name bar
because python is still using the old .pyc files from the foo package, none of which define bar. This can be especially problematic on a web server, where totally functioning code can break because of .pyc files.
As a result of both of these reasons (and possibly others), your deployment code and testing code should delete .pyc files, such as with the following line of bash:
find . -name '*.pyc' -delete
Also, as of python 2.6, you can run python with the -B flag to not use .pyc files. See How to avoid .pyc files? for more details.
See also: How do I remove all .pyc files from a project?
I am running a web app on python2.7 with mod_wsgi/apache. Everything is fine but I can't find any .pyc files. Do they not get generated with mod_wsgi?
By default apache probably doesn't have any write access to your django app directory which is a good thing security wise.
Now Python will byte recompile your code once every apache restart then cache it in memory.
As it is a longlive process it is ok.
Note: if you really really want to have those pyc, give a write access to your apache user to the source directory.
Note2: This can create a hell lot of confusion when you start with manage.py a test instance shared by apache as this will create those pyc as root and will keep them if you then run apache despite a source code change.
When a module is imported for the first time, or when the source is more recent than the current compiled file, a .pyc file containing the compiled code will usually be created in the same directory as the .py file.
So if you are not importing the module then no files will be created.
Besides this, a .pyc file may not be created is permissions problems with the directory. This can happen, for example, if you develop as one user but run as another, such as if you are testing with a web server. Creation of a .pyc file is automatic if you’re importing a module and Python has the ability (permissions, free space, etc.) to write the compiled module back to the directory.
Note - Running a script is not considered an import and no .pyc will be created.
If you need to create a .pyc file for a module that is not imported, you can use the py_compile and compileall modules.
The original file name is view.py, forms.py but python automatically adds/saves some file named view.pyc, forms.pyc
I want to delete those extra view.pyc and forms.pyc files.
Why do I get those .pyc files?
Or how can I tell python not to create those extra files with .pyc extension?
pyc files are compiled bytecode files, not backup files
6.1.2 ``Compiled'' Python files http://docs.python.org/release/1.5.1p1/tut/node43.html
As an important speed-up of the start-up time for short programs that use a lot of standard modules, if a file called "spam.pyc" exists in the directory where "spam.py" is found, this is assumed to contain an already-``byte-compiled'' version of the module spam. The modification time of the version of "spam.py" used to create "spam.pyc" is recorded in "spam.pyc", and the file is ignored if these don't match.
Update:
To avoid writing pyc files (Python 2.6+)
start with the -B flag (python -B)
set the PYTHONDONTWRITEBYTECODE environment variable.
See also How to avoid .pyc files?
Vim backups are named view.py~
Disable vim backups using
:se nobackup
.pyc files are compiled python modules created by the python interpreter; they have nothing at all to do with vim.
You can avoid them by invoking python with the -B flag or by setting the PYTHONDONTWRITEBYTECODE environment variable.
it's not vim, python compiles your py into that format to make things go faster.