original source : https://stackoverflow.com/a/60215

python은 switch구문이 없는데 이에 대한 우회방법이다.

I’ve always liked doing it this way

result = {
  'a': lambda x: x * 5,
  'b': lambda x: x + 7,
  'c': lambda x: x - 2
}[value](x)

From here

original source : https://youtu.be/-ARI4Cz-awo

참고자료) 19분 분량 중간 찾아서 봐도 된다. 실제 django 에서는 어떻게 사용되는지 보여준다. https://youtu.be/BsbtPqQdo3Q

내자신의 log를 출력하려는 경우 https://stackoverflow.com/a/47001420

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loggin level은 5가지이다. 기본적으로 WARNING이상만 출력된다. 그 아래의 경우는 logging.basicConfig(level=logging.DEBUG)와 같이 변경해야 출력 된다.

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log file에 출력하는 것을 보여준다.

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위는 formatter에 사용가능한 attributes들을 보여주고 있다.

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위 그림은 formatter의 사용을 예로 보여주고 있다.

original source : https://youtu.be/jxmzY9soFXg

아래 그림은 한 화일에서만 사용가능한 방법이다. import를 해서 다른 module을 가져오는 경우 import되어 들어오는 module에 logging이 설정되어있는 경우 엉키게 된다.

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위와 같이 사용해서 엉키는 문제는 아래와 같이 logger를 사용해서 해결한다.

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바로 아래 내용은 아래아래 내용과 같은 효과를 보여준다.

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아래는 file로도 logging내용을 출력하고 console에도 동시에 출력하는 것을 보여준다.

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original source: https://docs.djangoproject.com/en/3.0/ref/applications/#django.apps.AppConfig

Applications

(여기 doc에서는 app의 개념이다. 전체구성은 project라한다.)

Django contains a registry of installed applications that stores configuration and provides introspection. It also maintains a list of available models.

This registry is called apps and it’s available in django.apps:

>>> from django.apps import apps
>>> apps.get_app_config(‘admin’).verbose_name ‘Administration’

django는 각각의 설치된 app의 설정들을 apps이라는 registry(목록) 안에 보관하며 이를 통해 introspection이 가능하게 한다. 또한 사용가능한 model들을 관리한다.


Projects and applications

The term project describes a Django web application. The project Python package is defined primarily by a settings module, but it usually contains other things. For example, when you run django-admin startproject mysite you’ll get a mysite project directory that contains a mysite Python package with settings.py, urls.py, asgi.py and wsgi.py. The project package is often extended to include things like fixtures, CSS, and templates which aren’t tied to a particular application.

django-admin startproject 명령을 통해 만들어지는 것이 project이다. 이안에 settings.py, urls.py, asgi.py and wsgi.py. 이 있으며 fixtures, CSS, and templates 들도 포함된다.

A project’s root directory (the one that contains manage.py) is usually the container for all of a project’s applications which aren’t installed separately.

The term application describes a Python package that provides some set of features. Applications may be reused in various projects.

application은 python manage.py startapp 명령을 통해 만들어진다. 

Applications include some combination of models, views, templates, template tags, static files, URLs, middleware, etc. They’re generally wired into projects with the INSTALLED_APPS setting and optionally with other mechanisms such as URLconfs, the MIDDLEWARE setting, or template inheritance.


app은 보통 INSTALLED_APPS에 기입함으로써 프로젝트에 연결된다.

It is important to understand that a Django application is a set of code that interacts with various parts of the framework. There’s no such thing as an Application object. However, there’s a few places where Django needs to interact with installed applications, mainly for configuration and also for introspection. That’s why the application registry maintains metadata in an AppConfig instance for each installed application.

app은 obj로 존재하지 않는다. django가 app과 상호반응해야 하는데(주로 configuration, introspection의 목적으로) 그런 목적으로 apps registry의 AppConfig안에 metadata를 유지한다.  

There’s no restriction that a project package can’t also be considered an application and have models, etc. (which would require adding it to INSTALLED_APPS).

Configuring applications

To configure an application, subclass AppConfig and put the dotted path to that subclass in INSTALLED_APPS.

When INSTALLED_APPS contains the dotted path to an application module, Django checks for a default_app_config variable in that module.

INSTALLED_APPS 안에 . path 형태로 app 의 module경로가 지정되어있는 경우 해당 module의 __init__.py 안의 default_app_config 를 확인하게 된다.

If it’s defined, it’s the dotted path to the AppConfig subclass for that application.

default_app_config 안에는 .  path 형태로 AppConfig 를 implement한 subclass 위치가 지정되어야한다.

If there is no default_app_config, Django uses the base AppConfig class.

default_app_config allows applications that predate Django 1.7 such as django.contrib.admin to opt-in to AppConfig features without requiring users to update their INSTALLED_APPS.

이부분은 정확하게 이해하지는 않았다. 추측해보면 ‘default_app_config 를 설정해서 app을 만든경우 app을 사용하는 다른 사용자(개발자)가 INSTALLED_APPS에 기입하지 않고도 사용가능하다’ 인것 같다.

New applications should avoid default_app_config. Instead they should require the dotted path to the appropriate AppConfig subclass to be configured explicitly in INSTALLED_APPS.

app을 만들 경우 default_app_config 를 설정할 필요는 없으며 보통 INSTALLED_APPS에 AppConfig subclass 까지의 경로를 직접 . path형태로 기입하면 된다.

For application authors

If you’re creating a pluggable app called “Rock ’n’ roll”, here’s how you would provide a proper name for the admin:

# rock_n_roll/apps.py

from django.apps import AppConfig

class RockNRollConfig(AppConfig):
    name = 'rock_n_roll'
    verbose_name = "Rock ’n’ roll"

You can make your application load this AppConfig subclass by default as follows:

# rock_n_roll/__init__.py

default_app_config = 'rock_n_roll.apps.RockNRollConfig'

That will cause RockNRollConfig to be used when INSTALLED_APPS contains 'rock_n_roll'. This allows you to make use of AppConfig features without requiring your users to update their INSTALLED_APPS setting. Besides this use case, it’s best to avoid using default_app_config and instead specify the app config class in INSTALLED_APPS as described next.

Of course, you can also tell your users to put 'rock_n_roll.apps.RockNRollConfig' in their INSTALLED_APPS setting. You can even provide several different AppConfig subclasses with different behaviors and allow your users to choose one via their INSTALLED_APPS setting.

app 개발자는 두가지 선택지를 가진다. 하나는 default_app_config 를 설정함으로써 app의 사용자(다른개발자)가 INSTALLED_APPS에 특별히 기입하지않고도 app을 사용할수 있게 하는 것. 또다른 하나는 INSTALLED_APPS에 AppConfig를 extends한 subclass까지의 경로를 . path형태로 직접 입력하는 것이다. 결과적으로 두가지 방법 모두 AppConfig subclass를 거쳐 가게 된다. 

The recommended convention is to put the configuration class in a submodule of the application called apps. However, this isn’t enforced by Django.

You must include the name attribute for Django to determine which application this configuration applies to. You can define any attributes documented in the AppConfig API reference.

Note

If your code imports the application registry in an application’s __init__.py, the name apps will clash with the apps submodule. The best practice is to move that code to a submodule and import it. A workaround is to import the registry under a different name:

from django.apps import apps as django_apps

For application users

If you’re using “Rock ’n’ roll” in a project called anthology, but you want it to show up as “Jazz Manouche” instead, you can provide your own configuration:

# anthology/apps.py

from rock_n_roll.apps import RockNRollConfig

class JazzManoucheConfig(RockNRollConfig):
    verbose_name = "Jazz Manouche"

# anthology/settings.py

INSTALLED_APPS = [
    'anthology.apps.JazzManoucheConfig',
    # ...
]

Again, defining project-specific configuration classes in a submodule called apps is a convention, not a requirement.

Application configuration

class

AppConfig

[source]

Application configuration objects store metadata for an application. Some attributes can be configured in AppConfig subclasses. Others are set by Django and read-only.

Configurable attributes

AppConfig.

name

Full Python path to the application, e.g. 'django.contrib.admin'.

This attribute defines which application the configuration applies to. It must be set in all AppConfig subclasses.

It must be unique across a Django project.

AppConfig.

label

Short name for the application, e.g. 'admin'

This attribute allows relabeling an application when two applications have conflicting labels. It defaults to the last component of name. It should be a valid Python identifier.

It must be unique across a Django project.

AppConfig.

verbose_name

Human-readable name for the application, e.g. “Administration”.

This attribute defaults to label.title().

AppConfig.

path

Filesystem path to the application directory, e.g. '/usr/lib/pythonX.Y/dist-packages/django/contrib/admin'.

In most cases, Django can automatically detect and set this, but you can also provide an explicit override as a class attribute on your AppConfig subclass. In a few situations this is required; for instance if the app package is a namespace package with multiple paths.

Read-only attributes

AppConfig.

module

Root module for the application, e.g. <module 'django.contrib.admin' from 'django/contrib/admin/__init__.py'>.

AppConfig.

models_module

Module containing the models, e.g. <module 'django.contrib.admin.models' from 'django/contrib/admin/models.py'>.

It may be None if the application doesn’t contain a models module. Note that the database related signals such as pre_migrate and post_migrate are only emitted for applications that have a models module.

Methods

AppConfig.

get_models

()

[source]

Returns an iterable of Model classes for this application.

Requires the app registry to be fully populated.

AppConfig.

get_model

(model_name, require_ready=True)

[source]

Returns the Model with the given model_name. model_name is case-insensitive.

Raises LookupError if no such model exists in this application.

Requires the app registry to be fully populated unless the require_ready argument is set to False. require_ready behaves exactly as in apps.get_model().

AppConfig.

ready

()

[source]

Subclasses can override this method to perform initialization tasks such as registering signals. It is called as soon as the registry is fully populated.

Although you can’t import models at the module-level where AppConfig classes are defined, you can import them in ready(), using either an import statement or get_model().

If you’re registering model signals, you can refer to the sender by its string label instead of using the model class itself.

Example:

from django.apps import AppConfig
from django.db.models.signals import pre_save


class RockNRollConfig(AppConfig):
    # ...

    def ready(self):
        # importing model classes
        from .models import MyModel  # or...
        MyModel = self.get_model('MyModel')

        # registering signals with the model's string label
        pre_save.connect(receiver, sender='app_label.MyModel')

Warning

Although you can access model classes as described above, avoid interacting with the database in your ready() implementation. This includes model methods that execute queries (save(), delete(), manager methods etc.), and also raw SQL queries via django.db.connection. Your ready() method will run during startup of every management command. For example, even though the test database configuration is separate from the production settings, manage.py test would still execute some queries against your production database!

Note

In the usual initialization process, the ready method is only called once by Django. But in some corner cases, particularly in tests which are fiddling with installed applications, ready might be called more than once. In that case, either write idempotent methods, or put a flag on your AppConfig classes to prevent re-running code which should be executed exactly one time.

Namespace packages as apps

Python packages without an __init__.py file are known as “namespace packages” and may be spread across multiple directories at different locations on sys.path (see PEP 420).

Django applications require a single base filesystem path where Django (depending on configuration) will search for templates, static assets, etc. Thus, namespace packages may only be Django applications if one of the following is true:

  1. The namespace package actually has only a single location (i.e. is not spread across more than one directory.)
  2. The AppConfig class used to configure the application has a path class attribute, which is the absolute directory path Django will use as the single base path for the application.

If neither of these conditions is met, Django will raise ImproperlyConfigured.

Application registry

apps

The application registry provides the following public API. Methods that aren’t listed below are considered private and may change without notice.

apps.

ready

Boolean attribute that is set to True after the registry is fully populated and all AppConfig.ready() methods are called.

apps.

get_app_configs

()

Returns an iterable of AppConfig instances.

apps.

get_app_config

(app_label)

Returns an AppConfig for the application with the given app_label. Raises LookupError if no such application exists.

apps.

is_installed

(app_name)

Checks whether an application with the given name exists in the registry. app_name is the full name of the app, e.g. 'django.contrib.admin'.

apps.

get_model

(app_label, model_name, require_ready=True)

Returns the Model with the given app_label and model_name. As a shortcut, this method also accepts a single argument in the form app_label.model_name. model_name is case-insensitive.

Raises LookupError if no such application or model exists. Raises ValueError when called with a single argument that doesn’t contain exactly one dot.

Requires the app registry to be fully populated unless the require_ready argument is set to False.

Setting require_ready to False allows looking up models while the app registry is being populated, specifically during the second phase where it imports models. Then get_model() has the same effect as importing the model. The main use case is to configure model classes with settings, such as AUTH_USER_MODEL.

When require_ready is False, get_model() returns a model class that may not be fully functional (reverse accessors may be missing, for example) until the app registry is fully populated. For this reason, it’s best to leave require_ready to the default value of True whenever possible.

Initialization process

How applications are loaded

When Django starts, django.setup() is responsible for populating the application registry.

setup

(set_prefix=True)

[source]

Configures Django by:

  • Loading the settings.
  • Setting up logging.
  • If set_prefix is True, setting the URL resolver script prefix to FORCE_SCRIPT_NAME if defined, or / otherwise.
  • Initializing the application registry.

This function is called automatically:

  • When running an HTTP server via Django’s WSGI support.
  • When invoking a management command.

It must be called explicitly in other cases, for instance in plain Python scripts.

The application registry is initialized in three stages. At each stage, Django processes all applications in the order of INSTALLED_APPS.

First Django imports each item in INSTALLED_APPS.

If it’s an application configuration class, Django imports the root package of the application, defined by its name attribute. If it’s a Python package, Django creates a default application configuration.

At this stage, your code shouldn’t import any models!

In other words, your applications’ root packages and the modules that define your application configuration classes shouldn’t import any models, even indirectly.

Strictly speaking, Django allows importing models once their application configuration is loaded. However, in order to avoid needless constraints on the order of INSTALLED_APPS, it’s strongly recommended not import any models at this stage.

Once this stage completes, APIs that operate on application configurations such as get_app_config() become usable.

Then Django attempts to import the models submodule of each application, if there is one.

You must define or import all models in your application’s models.py or models/__init__.py. Otherwise, the application registry may not be fully populated at this point, which could cause the ORM to malfunction.

Once this stage completes, APIs that operate on models such as get_model() become usable.

Finally Django runs the ready() method of each application configuration.

Troubleshooting

Here are some common problems that you may encounter during initialization:

AppRegistryNotReady: This happens when importing an application configuration or a models module triggers code that depends on the app registry.

For example, gettext() uses the app registry to look up translation catalogs in applications. To translate at import time, you need gettext_lazy() instead. (Using gettext() would be a bug, because the translation would happen at import time, rather than at each request depending on the active language.)

Executing database queries with the ORM at import time in models modules will also trigger this exception. The ORM cannot function properly until all models are available.

This exception also happens if you forget to call django.setup() in a standalone Python script.

ImportError: cannot import name ... This happens if the import sequence ends up in a loop.

To eliminate such problems, you should minimize dependencies between your models modules and do as little work as possible at import time. To avoid executing code at import time, you can move it into a function and cache its results. The code will be executed when you first need its results. This concept is known as “lazy evaluation”.

django.contrib.admin automatically performs autodiscovery of admin modules in installed applications. To prevent it, change your INSTALLED_APPS to contain 'django.contrib.admin.apps.SimpleAdminConfig' instead of 'django.contrib.admin'.

ORIGINAL SOURCE : https://seddonym.me/2018/06/12/installed-apps/

If you’ve ever built anything using Django, you’ll be used to including apps in your INSTALLED_APPS setting:

# settings.py

INSTALLED_APPS = [
    'django.contrib.auth',
    'myapp',
]

This is something we take for granted, but it’s not always obvious why we need to do this. For something we use so routinely, it’s a concept we’re surprisingly vague about.

So, what are Django apps for? Why do we need to install them? Why can’t they just be plain old Python packages? Before you read on, think for a moment and try to answer these questions for yourself.

What apps are for: a general rule

Here’s a general rule for the purpose of a Django app:

Apps are for introspection.

When you declare a particular Python package as an app (by including it in INSTALLED_APPS), you allow your project to ‘look into itself’ (i.e. introspect) and discover and enable functionality within that package.

By way of contrast, look at an aspect of Django that doesn’t use the apps system: middleware. Middleware classes can sit anywhere in your project, but they need to be explicitly configured:

MIDDLEWARE = [
    'myapp.middleware.MyMiddleware',
]

In contrast, features which use the apps system (e.g. models) don’t need any specific configuration to let the framework know about them other than app installation.

Some examples

  1. Models: The most obvious one; Django looks for any models module within your package and handles the necessary database migrations.
  2. Admin: You register models in the Django Admin Site within an admin module.
  3. Template tags: You may include a templatetags package to allow loading of custom template tags.
  4. Template directories: Assuming you have configured your TEMPLATES setting with APP_DIRS as True, Django will know about any files within a templates subdirectory in your app.
  5. Celery tasks: If you’re using Celery, you can include your tasks in a tasks.py and Celery will automatically pick them up.

Autodiscovery

Notice that all of these examples follow a similar pattern: for each piece of functionality, Django looks in every installed app for a particular module/directory name; if it’s there, it will use it. This is the foundation of Django’s pluggable architecture; providing a package is in your Python path, adding it to INSTALLED_APPS allows it to hook in to other parts of the system.

This process is called autodiscovery, and there is a built in function for it: django.utils.module_loading.autodiscover_modules. To use, simply call it with the name of the module you want, and Django will import any modules that have that name (providing they are in INSTALLED_APPS, of course). For example, to import every module named foo.py, just call:

from django.utils.module_loading import autodiscover_modules

autodiscover_modules('foo')

Tip

A good place to call autodiscover_modules is in the ready() method of your application configuration class.

# myapp/__init__.py

from django.apps import AppConfig as BaseAppConfig
from django.utils.module_loading import autodiscover_modules


class AppConfig(BaseAppConfig):
    name = 'myapp'

    def ready(self):
        autodiscover_modules('products')


default_app_config = 'myapp.AppConfig'
    

Understanding INSTALLED_APPS can improve your Django

Why is knowing this useful? If you understand the fundamental concept of what an app is, you should be able to structure your projects better. Apps should be pluggable, discoverable modules, so design them accordingly.

That means if you have a large amount of interdependent functionality, there really is no need to split it up into multiple apps. You’re probably better off splitting up the code into a manageable file structure. For example, there’s nothing special about models.py other than it gets automatically imported during certain model-related tasks. To make things more manageable, you could turn models.py into a subpackage within your app, distribute the models within different files inside it, and import those models within the models/__init__.py.

Conversely, if parts of your project could be designed as plugins, consider making them apps (even if they don’t declare any models). Let’s say you have a site that defines a range of different products by inheriting from a base Product class. Rather than manually importing them, or configuring them in your settings, you could have your core products app autodiscover modules named products. This would allow you to have separate, per-product apps that declare their products in products.py files. (For more ideas about this, you may like to watch my talk on Encapsulated Django.)

To sum up, the concept of a Django app is a powerful one that goes beyond mere models. It’s the magic that allows the framework to find out about what’s inside your project. Knowing this can help you understand how certain things get imported, and give you confidence to be inventive in the way you architect your code.

original source : https://youtu.be/N5vscPTWKOk

pip

   Pip is a package management system used to install and manage software packages, such as those found in the Python Package Index.

vitualenv : 

virtualenv is used to manage Python packages for different projects. Using virtualenv allows you to avoid installing Python packages globally which could break system tools or other projects. You can install virtualenv using pip. 각각의 project별로 필요한 dependencies가 다르므로 각기 다른 환경에서 작업 할수 있게 해주는 도구이다. 

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참고사항 : https://youtu.be/nxOPY1eFL9A

==========================================================…original source : https://www.youtube.com/playlist?list=PLS1QulWo1RIYt4e0WnBp-ZjCNq8X0FX0J

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자바, kotlin의 경우 여러개의 생성자를 정의할수 있다. 

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위위의 myfunctions.py 모듈을 import로 가져와서 사용하는 것을 보여주고 있다.

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위는 모듈이 다른 directory에 있을 때를 보여주고 있다.

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위는 모듈이 다른 directory에 있을 때의 또 다른 방법을 보여주고 있다

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하나의 class 가 하나의 class안에 들어가는 것을 composition 이라고 한다. 

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writing file

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reading files

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아래와 같이 없는 화일의 이름으로 여는 경우 filenotfounderror가 발생한다.

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json file 읽고 쓰기

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iterator

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generators

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command line arguments

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lambda 

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nested function and closure

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decorator

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operator overloading

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global, local, nonlocal 변수

Iterate over object attributes in python