Django 玩转API

 

现在，让我们进入Python的交互式shell，玩转这些Django提供给你的API。 使用如下命令来调用Python shell：

$ python manage.py shell

我们使用上述命令而不是简单地键入“python”进入python环境，是因为manage.py 设置了DJANGO_SETTINGS_MODULE 环境变量，该变环境变量告诉Django导入mysite/settings.py文件的路径。

绕开 manage.py

如果你不想使用manage.py，也没问题。只要设置DJANGO_SETTINGS_MODULE 环境变量为 mysite.settings，启动一个普通的Python shell，然后建立Django：

>>> import django
>>> django.setup()

如果以上命令引发了一个AttributeError，可能是你使用了一个和本教程不匹配的Django版本。 你可能需要换一个老一点的教程或者换一个新一点的Django版本。

你必须在与manage.py相同的目录下运行python，或确保你的目录在Python 的路径中，这样import mysite才可以工作。

所有这些信息，请参见django-admin 的文档。

一旦你进入这个shell，请探索这些数据库 API：

>>> from polls.models import Question, Choice   # Import the model classes we just wrote.

# No questions are in the system yet.
>>> Question.objects.all()
[]

# Create a new Question.
# Support for time zones is enabled in the default settings file, so
# Django expects a datetime with tzinfo for pub_date. Use timezone.now()
# instead of datetime.datetime.now() and it will do the right thing.
>>> from django.utils import timezone
>>> q = Question(question_text="What's new?", pub_date=timezone.now())

# Save the object into the database. You have to call save() explicitly.
>>> q.save()

# Now it has an ID. Note that this might say "1L" instead of "1", depending
# on which database you're using. That's no biggie; it just means your
# database backend prefers to return integers as Python long integer
# objects.
>>> q.id
1

# Access model field values via Python attributes.
>>> q.question_text
"What's new?"
>>> q.pub_date
datetime.datetime(2012, 2, 26, 13, 0, 0, 775217, tzinfo=<UTC>)

# Change values by changing the attributes, then calling save().
>>> q.question_text = "What's up?"
>>> q.save()

# objects.all() displays all the questions in the database.
>>> Question.objects.all()
[<Question: Question object>]

先等一下。<Question: Question object>对于这个对象是一个完全没有意义的表示。 让我们来修复这个问题，编辑Question模型（在polls/models.py文件中）并添加一个__str__()方法给Question和Choice：

polls/models.py

from django.db import models

class Question(models.Model):
    # ...
    def __str__(self):              # __unicode__ on Python 2
        return self.question_text

class Choice(models.Model):
    # ...
    def __str__(self):              # __unicode__ on Python 2
        return self.choice_text

给你的模型添加__str__()方法很重要，不仅会使你自己在使用交互式命令行时看得更加方便，而且会在Django自动生成的管理界面中使用对象的这种表示。

__str__ 还是 __unicode__?

对于Python 3来说，这很简单，只需使用__str__()。

对于Python 2来说，你应该定义__unicode__()方法并返回unicode 值。Django 模型具有一个默认的__str__() 方法，它会调用__unicode__()并将结果转换为UTF-8 字节字符串。这意味着unicode(p)将返回一个Unicode 字符串，而str(p)将返回一个字节字符串，其字符以UTF-8编码。Python 的行为则相反：对象的__unicode__方法调用 __str__方法并将结果理解为ASCII 字节字符串。这个不同点可能会产生困惑。

如果以上这些令你费解的话，那就使用Python 3吧。

请注意这些都是普通的Python方法。 让我们演示一下如何添加一个自定义的方法：

polls/models.py

import datetime

from django.db import models
from django.utils import timezone


class Question(models.Model):
    # ...
    def was_published_recently(self):
        return self.pub_date >= timezone.now() - datetime.timedelta(days=1)

注意import datetime 和from django.utils import timezone分别引用Python 的标准datetime 模块和Djangodjango.utils.timezone中时区相关的工具。如果你不了解Python中时区的处理方法，你可以在时区支持的文档 中了解更多的知识。

保存这些改动，然后通过python manage.py shell再次打开一个新的Python 交互式shell：

>>> from polls.models import Question, Choice

# Make sure our __str__() addition worked.
>>> Question.objects.all()
[<Question: What's up?>]

# Django provides a rich database lookup API that's entirely driven by
# keyword arguments.
>>> Question.objects.filter(id=1)
[<Question: What's up?>]
>>> Question.objects.filter(question_text__startswith='What')
[<Question: What's up?>]

# Get the question that was published this year.
>>> from django.utils import timezone
>>> current_year = timezone.now().year
>>> Question.objects.get(pub_date__year=current_year)
<Question: What's up?>

# Request an ID that doesn't exist, this will raise an exception.
>>> Question.objects.get(id=2)
Traceback (most recent call last):
    ...
DoesNotExist: Question matching query does not exist.

# Lookup by a primary key is the most common case, so Django provides a
# shortcut for primary-key exact lookups.
# The following is identical to Question.objects.get(id=1).
>>> Question.objects.get(pk=1)
<Question: What's up?>

# Make sure our custom method worked.
>>> q = Question.objects.get(pk=1)
>>> q.was_published_recently()
True

# Give the Question a couple of Choices. The create call constructs a new
# Choice object, does the INSERT statement, adds the choice to the set
# of available choices and returns the new Choice object. Django creates
# a set to hold the "other side" of a ForeignKey relation
# (e.g. a question's choice) which can be accessed via the API.
>>> q = Question.objects.get(pk=1)

# Display any choices from the related object set -- none so far.
>>> q.choice_set.all()
[]

# Create three choices.
>>> q.choice_set.create(choice_text='Not much', votes=0)
<Choice: Not much>
>>> q.choice_set.create(choice_text='The sky', votes=0)
<Choice: The sky>
>>> c = q.choice_set.create(choice_text='Just hacking again', votes=0)

# Choice objects have API access to their related Question objects.
>>> c.question
<Question: What's up?>

# And vice versa: Question objects get access to Choice objects.
>>> q.choice_set.all()
[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
>>> q.choice_set.count()
3

# The API automatically follows relationships as far as you need.
# Use double underscores to separate relationships.
# This works as many levels deep as you want; there's no limit.
# Find all Choices for any question whose pub_date is in this year
# (reusing the 'current_year' variable we created above).
>>> Choice.objects.filter(question__pub_date__year=current_year)
[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]

# Let's delete one of the choices. Use delete() for that.
>>> c = q.choice_set.filter(choice_text__startswith='Just hacking')
>>> c.delete()

更多关于模型关联的信息，请查看访问关联的对象。更多关于如何在这些API中使用双下划线来执行字段查询的信息，请查看 字段查询。更多关于数据库API的信息，请查看我们的 数据库API参考。

如果你适应了这些API，可以阅读本教程的第2部分来让Django的自动管理界面工作起来。

########################################################

1. def __str__(self):              # __unicode__ on Python 2
内置函数 __str__(), __unicode__()的作用(Python 2用的是__unicode__, python 3用的是__str__):
__str__()用于表示对象代表的含义，返回一个字符串.实现了__str__()方法后，可以直接使用print语句输出对象，也可以通过函数str()触发__str__()的执行.这样就把对象和字符串关联起来，便于某些程序的实现，可以用这个字符串来表示某个类

#!/usr/bin/python
# -*- coding: UTF-8 -*-
class Fruit:      
    '''Fruit类'''               #为Fruit类定义了文档字符串
    def __str__(self):          # 定义对象的字符串表示
        return self.__doc__

if __name__ == "__main__":
    fruit = Fruit()
    print str(fruit)            # 调用内置函数str()出发__str__()方法，输出结果为:Fruit类
    print fruit                 #直接输出对象fruit,返回__str__()方法的值，输出结果为:Fruit类

2.pub_date__year
一个属性后面跟__year过滤。
3.

>>> q = Question.objects.get(pk=1)
>>> q.choice_set.all()
[]
# Create choices.
>>> q.choice_set.create(choice_text='The sky', votes=0)   
<Choice: The sky>

question和choice是一对多的关系。q是question对象，q.choice即是q对应的choice， q.choice_set调用了choice对象下内置的_set, q.choice_set.create是创建choice的方法。