Is it possible to dynamically create classes in the __init__.py file.
I'll give an example. Let's say I have this hierarchy:
script.py
module\
--> __init__.py
In the python file script.py, I want to know if I can, with this call:
from module import my_class
dynamically create the my_class definition in the __init__.py
Related
How can one get the fully qualified name of a module in Python, at the module level?
For example, let's say this is the file structure:
foo
├── setup.py
└── bar
└── baz
└── spam.py
From within the spam.py file at the module level, how can I get the fully qualified module name "bar.baz.spam"?
There is __file__, which one can pair with inspect.getmodulename(__file__) to get the module name "spam". However, how can I turn this into "bar.baz.spam"?
Try something like this:
from bar.baz import spam
print(spam.__name__)
If the name being imported is not a module, you can get the module name and object name like this:
from bar.baz.spam import MyClass
print(MyClass.__module__, MyClass.__name__)
I am working on a program that is structured in the following way:
program/
base/
__init__.py
module1.py
module2.py
...
app1/
__init__.py
module11.py
module12.py
...
app2/
__init__.py
module21.py
module22.py
...
...
Package base's __init__.py is used to expose a minimum set of objects collectively required by app1 and app2, like so:
# __init__ file
from module1 import rock, paper
from module2 import scissor
...
I understand that when a program implements a class, and when that program is executed, the interpreter calls the class's __new__ method if it exists, then calls __init__ method.
My question is this: can packages implement, say, a __new__.py file that works in the same way as a class's __new__ method? In the case of my program, I would be using such a file to dynamically alter rock, paper, or scissor.
No, they can't. The specification of the import system declares that only __init__.py is used in a special manner by the language. Any other file you put in the folder is just a regular file.
The similarities with the __init__ method in object creation are limited to the name.
Say I have ClassA, ClassB and ClassC each within their own .py files called ClassA.py, ClassB.py and ClassC.py respectively.
# Inside ClassA.py
class ClassA:
pass
In my __init__.py I end up with
from .ClassA import ClassA
from .ClassB import ClassB
from .ClassC import ClassC
Every time I add a new class (using the same structure), I have to change the __init__.py. Is there a recommended way to automate this, given that all files in this package follow the same (not-quite-pythonic-but-oh-well) structure?
It is possible, but not recommended.
Consider the following directory structure:
├── test_module
│ ├── ClassA.py
│ ├── ClassB.py
│ └── __init__.py
We want to import ClassA.ClassA and ClassB.ClassB into __init__.py programmatically.
Assume these are the contents of ClassA.py:
class ClassA:
pass
ClassB.py is identical, save for the name of the class.
Now, say that in __init__.py we want to traverse the root directory non-recursively (not that it matters, since we have no directories) and import all classes from all modules within.
__init__.py:
import os
from importlib import import_module
my_location = os.path.dirname(__file__)
module_list = [file
for file in os.listdir(my_location)
if os.path.splitext(file)[1] == '.py'
and file != '__init__.py']
modules = [import_module(f'.{os.path.splitext(module)[0]}', __name__)
for module in module_list]
After running import test_module, the classes from ClassA.py and ClassB.py will be imported into the working namespace as test_module.ClassA.ClassA and test_module.ClassB.ClassB respectively.
To illustrate:
>>> import test_module
>>> test_module.ClassA.ClassA()
<test_module.ClassA.ClassA object at 0x7f1e66181fd0>
For completeness, if you want this script to mimic the behaviour of from X import Y:
globals().update({name: getattr(module, name)
for module in modules
for name in module.__dict__
if not name.startswith('_')})
Importing:
>>> import test_module
>>> test_module.ClassA()
<test_module.ClassA.ClassA object at 0x7fb8edb9dfd0>
These will make those names accessible as test_module.ClassA etc. (because you are importing from test_module, we add an additional layer of indirection. In test_module's scope, they are accessible directly as unqualified name.
There are additional bells and whistles we could add, such as checking each module's __all__ attribute and performing recursive traversal of sub-directories, but that is rather out of scope of this question, and I must emphasise that to my mind, it would be better to refactor your code such that this is not needed in the first place, rather than tack on functionality that plays with the internals of Python where it does not seem to be essential.
Yes, you can. This is what the importlib package is for. It contains the lower level calls that actually do the importing. It lets you do imports by calling the methods it provides.
So you could use Python os.path.listdir or whatever to learn what your subpackages are, and then make calls to importlib to import each of them.
I haven't done much of this myself. Just enough to know this exists. Maybe someone else can give you more details. Also, this link might be helpful to you:
https://dev.to/0xcrypto/dynamic-importing-stuff-in-python--1805
I am attempting to create a package (mypackage) that contains a few classes, but would like the classes contained in multiple files.
For example, I want class_a.py to contain a class named ClassA, etc...
Thus, I would like the following file structure:
.../mypackage
__init__.py
class_a.py
class_b.py
...
However, I would like to load and use the package as follows:
load mypackage
a = mypackage.ClassA()
What do I need to do (I assume in the __init__.py) file to make this possible. Currently, it operates using "mypackage.class_a.ClassA()"?
As mentioned, in your __init__.py for a class, use the following:
from class_a import ClassA
from class_b import ClassB
for the case of a file without a class, use the following:
from . import file_a
from . import file_b
or if you only want to expose specific methods of a file:
from .file_a import method_a
from .file_b import method_b
Make your __init__.py import all your ClassA, ClassB, etc from other files.
Then you'll be able to import mypackage and use mypackage.ClassA, or from mypackage import ClassA and use it as unqualified ClassA.
A bit of background.
An import foo statement looks for foo.py, then for foo/__init__.py, and loads the names defined in that file into the current namespace. Put whatever you need to be "top-level" into __init__.py.
Also, take a look at __all__ top-level variable if you tend to from mypackage import *.
In your __init__.py, add this:
from class_a import ClassA
from class_b import ClassB
del class_a
del class_b
I have a python package main and other_python_files which are like:
main/
__init__.py
lib.py
other_python_files/
__init__.py
test.py
Let lib.py contain a class called MyClass. When I do from main import lib.py and use MyClass inside test.py I get the error that MyClass is not defined.
I tried doing from main import MyClass inside the init file in the main directory but I still get the same error. What should I do to achieve importing a specific class from the lib.py file ?
You either have to import that class out of lib:
from main.lib import MyClass
Or use lib.MyClass in place of MyClass.
You can also import MyClass inside of the __init__.py file that's in main, which lets you import it the way you originally tried:
__all__ = ['MyClass']
from lib import MyClass
You can read about __all__ here: Can someone explain __all__ in Python?