I created two classes. The first class takes an image from the working directory and then covert the image from pdf to jpg using wand. The second class takes the created jpg image and then do further manipulations with the image.
Now When I try to run the first class and then the second class right after that; python crashes because the second class is trying to look for the image but it wont find it until it is created.
My question is how can you run the second class but just after the first class is executed.
class1 = imagecreation('image.jpg')
class2 = transformimage()
I found the answer to my question. When you reference classes and you import as a module in a different one; the module will get call when is defined if you do not use if __name__=='__main__':. By putting this code at the end of the code will only execute the module once it is intended to be executed but not when you import the module. This way you can use the modules by themselves and also to import from other modules.
Related
I am trying to load a class at run time from a configuration file. The module that contains the class will contain many other classes and I don't want to import them all. The pattern given in this question Import class from module dynamically
cls = getattr(import_module('my_module'), 'my_class')
loads in the entire module which is exactly what I am trying to avoid. Is there way to get just 'my_class' without everything else in 'my_module'?
I'm developing a small app using kivy and python3.6 (I'm still a beginner). I'm planning to separate the code in different files for clarity, however I have encountered a problem in a specific situation. I have made minimal working example to illustrate.
I have the following files:
main.py
main.kv
module.py
module.kv
Here a minimal code:
main.py:
from kivy.app import App
from kivy.uix.button import Button
from kivy.lang import Builder
import module
Builder.load_file('module.kv')
class MainApp(App):
pass
def function():
print('parent function')
if __name__ == '__main__':
MainApp().run()
main.kv:
CallFunction
module.py:
from kivy.uix.button import Button
class CallFunction(Button):
def call_function(self):
from main import function
function()
module.kv:
<CallFunction>:
id : parent_button
text: 'Call parent button'
on_press: self.call_function()
So the problem is that when I run this code, I receive a warning
The file /home/kivy/python_exp/test/module.kv is loaded multiples times, you might have unwanted behaviors.
What works:
If the function I want to call is part of the main app class, there is no problem
If the function is part of the module.py there is no problem
If the function is part of another module, there is no problem
What doesn't work
I cannot call a function which is in the main.py. If I use the import the function as the beginning of module.py, kivy has a weird behavior and call everything twice. Calling within this call_function allows to have a proper interface, but I get the warning that the file has been loaded multiple time.
There are easy workarounds, I'm well aware of that, so it's more about curiosity and understanding better how the imports in kivy works. Is there a way to make it work?
I wanted to use the main.py to initialize different things at the startup of the app. In particular I wanted to create an instance of another class (not a kivy class) in the main.py and when clicking on the button on the interface, calling a method on this instance.
Thanks :)
When you import something from another python module the python virtual machine execute this module. In the call_function you import function from the main file so everytime you press the CallFunction the module.kv is loaded.
To solve this it is recommended to include the other kv files in your main kv file.
You can also move the import statement from the method to the top of the module file.
Your kv file is loaded twice because the code is executed twice. This is due to how pythons module system works and kivy just realized that loading the kv twice is probably not what you want.
Generally python objects live in a namespace. So when a function in the module foo looks up a variable the variable is searched in the namespace of the module. That way if you define two variables foo.var and bar.var (in the modules foo and bar resp.) they don't clash and get confused for each other.
The tricky thing is that the python file you execute is special: It does not create a module namespace but the __main__ namespace. Thus if you import the file you are executing as __main__ it will create a whole new namespace with new objects and execute the module code. If you import a module that was already imported in the current session the module code is not executed again, but the namespace already created is made available. You don't even need two files for that, put the following in test.py:
print("hello!")
print(__name__)
import test
If you now execute python test.py you will see two hello! and once __main__ and once test.
You can find more information on namespaces and how variable lookups works in python in the documentation.
Also if your function actually does some work and mutates an object that lives in main.py you might want to rethink the information flow. Often it is a good idea to bind the state and functions working on them together in classes and passing the objects then where they are called i.e. to CallFunction in your example.
I am a new comer when it comes to package and module to python.
I am trying to cut my script in several separate compartiment. to improve readability and maintenance.
My problem is the following:
I have a module which define a class and a function inside this module which instantiate this class.
module blast.py
class Blast():
blabla
def foo():
blast = Blast()
# do some stuff
this module is inside a package with a _ _init__.py file
__all__ = ["blast"]
I have a main script In which i want to use that function.
I import the module with
from package import blast
But To use that function I have to use the name space of the module ( at least my IDE say me that: pycharm)
blast.foo()
So does it works? does the function will see the class inside it module?
And more generally Could I import some function of my package inside my namespace. I though it was done this way and answer I got from internet doesn't really help me.
Yes, the function blast.foo() would know and find the class Blast.
Whenever you import a module, in part or in its entirety, the entire module is loaded - the way you import it merely decides on what classes and functions are available in the current scope, and in what way.
For example, if you call this:
from package.blast import foo
only the function foo() would be available, but the entire package read and loaded. if you were to try instantiate the class Blast by itself in the same script, this would not work.
By the way, you can make importing functions more convenient, if you customize __init__.py. In your case, if you were to edit it like so:
>>>__init__.py
from blast.py import Blast, foo
you can import both function and class like so:
from package import Blast, foo
The reason why you __all__ parameter does not work, is because it requires another import statement - mainly this:
from package import *
Calling this with your current __init__.py should work as expected.
Perhaps this post by Mike Grouchy is able to clarify things a bit more.
I am confused about some behavior of Python. I always thought importing a module basically meant executing it. (Like they say here: Does python execute imports on importation) So I created three simple scripts to test something:
main.py
import config
print(config.a)
config.a += 1
print(config.a)
import test
print(config.a)
config.py
def get_a():
print("get_a is called")
return 1
a = get_a()
test.py
import config
print(config.a)
config.a += 1
The output when running main.py is:
get_a is called
1
2
2
3
Now I am confused because I expected get_a() to be called twice, once from main.py and once from test.py. Can someone please explain why it is not? What if I really wanted to import config a second time, like it was in the beginning with a=1?
(Fortunately, for my project this behavior is exactly what I wanted, because get_a() corresponds to a function, which reads lots of data from a database and of course I only want to read it once, but it should be accessible from multiple modules.)
Because the config module is already loaded so there's no need to 'run' it anymore, just return the loaded instance.
Some standard library modules make use of this, from example random. It creates an object of class Random on first import and reuses it when it gets imported again. A comment on the module reads:
# Create one instance, seeded from current time, and export its methods
# as module-level functions. The functions share state across all uses
#(both in the user's code and in the Python libraries), but that's fine
# for most programs and is easier for the casual user than making them
# instantiate their own Random() instance.
I am modifying a module which contains a class in it.
When I %run another module that uses the mofified class from IPython, the changes do not seem to take effect unless I restart IPython.
I have tried to use imp.reload, but this does not help. For example, I have put the following the code at the top of my module, but it does not appear to be using the updated version of my modified class (BigMySQLDatabaseGetter in the big_mysql_database_getter module)
import imp
import sys
from big_mysql_database_getter import BigMySQLDatabaseGetter
module_big_mysql_database_getter = sys.modules['big_mysql_database_getter']
imp.reload(module_big_mysql_database_getter)
Reloading a module doesn't automatically update all references that were created before, it just redefines everything within the module.
So if you do something like:
from spam import eggs
imp.reload(spam)
print(spam.eggs is eggs)
you'll get False, as eggs still references the old class. Likewise, instances created before the reload are instances of the old class, not of the new class:
import spam
e = spam.eggs()
imp.reload(spam)
print(isinstance(e, spam.eggs)) # False!
In your case, you can either reimport BigMySQLDatabaseGetter after reloading the module, or instead of directly importing the class just import the module and use big_mysql_database_getter.BigMySQLDatabaseGetter instead.