在Django的视图中使用form对象的方法
在学习了关于Form类的基本知识后，你会看到我们如何把它用到视图中，取代contact()代码中不整齐的部分。 一下示例说明了我们如何用forms框架重写contact()：
# views.py
from django.shortcuts import render_to_response
from mysite.contact.forms import ContactForm
def contact(request):
if request.method == 'POST':
form = ContactForm(request.POST)
if form.is_valid():
cd = form.cleaned_data
send_mail(
cd['subject'],
cd['message'],
cd.get('email', 'noreply@example.com'),
['siteowner@example.com'],
)
return HttpResponseRedirect('/contact/thanks/')
else:
form = ContactForm()
return render_to_response('contact_form.html', {'form': form})
# contact_form.html
<html>
<head>
<title>Contact us</title>
</head>
<body>
<h1 id="Contact-us">Contact us</h1>
{% if form.errors %}
<p style="color: red;">
Please correct the error{{ form.errors|pluralize }} below.
</p>
{% endif %}
<form action="" method="post">
<table>
{{ form.as_table }}
</table>
<input type="submit" value="Submit">
</form>
</body>
</html>
看看，我们能移除这么多不整齐的代码！ Django的forms框架处理HTML显示、数据校验、数据清理和表单错误重现。
尝试在本地运行。 装载表单，先留空所有字段提交空表单；继而填写一个错误的邮箱地址再尝试提交表单；最后再用正确数据提交表单。 （根据服务器的设置，当send_mail()被调用时，你将得到一个错误提示。而这是另一个问题。）
改变字段显示
你可能首先注意到：当你在本地显示这个表单的时，message字段被显示成`` input type=”text”`` ，而它应该被显示成<`` textarea`` >。我们可以通过设置* widget* 来修改它：
from django import forms class ContactForm(forms.Form): subject = forms.CharField() email = forms.EmailField(required=False) message = forms.CharField(**widget=forms.Textarea** )
forms框架把每一个字段的显示逻辑分离到一组部件（widget）中。 每一个字段类型都拥有一个默认的部件，我们也可以容易地替换掉默认的部件，或者提供一个自定义的部件。
考虑一下Field类表现* 校验逻辑* ，而部件表现* 显示逻辑* 。
设置最大长度
一个最经常使用的校验要求是检查字段长度。 另外，我们应该改进ContactForm，使subject限制在100个字符以内。 为此，仅需为CharField提供max_length参数，像这样：
from django import forms class ContactForm(forms.Form): subject = forms.CharField(**max_length=100** ) email = forms.EmailField(required=False) message = forms.CharField(widget=forms.Textarea)
选项min_length参数同样可用。
设置初始值
让我们再改进一下这个表单：为字subject段添加* 初始值* ： "I love your site!" （一点建议，但没坏处。）为此，我们可以在创建Form实体时，使用initial参数：
def contact(request):
if request.method == 'POST':
form = ContactForm(request.POST)
if form.is_valid():
cd = form.cleaned_data
send_mail(
cd['subject'],
cd['message'],
cd.get('email', `'noreply@example.com`_'),
[`'siteowner@example.com`_'],
)
return HttpResponseRedirect('/contact/thanks/')
else:
form = ContactForm(
**initial={'subject': 'I love your site!'}**
)
return render_to_response('contact_form.html', {'form': form})
现在，subject字段将被那个句子填充。
请注意，传入* 初始值* 数据和传入数据以* 绑定* 表单是有区别的。 最大的区别是，如果仅传入* 初始值* 数据，表单是unbound的，那意味着它没有错误消息。
自定义校验规则
假设我们已经发布了反馈页面了，email已经开始源源不断地涌入了。 这里有一个问题： 一些提交的消息只有一两个字，我们无法得知详细的信息。 所以我们决定增加一条新的校验： 来点专业精神，最起码写四个字，拜托。
我们有很多的方法把我们的自定义校验挂在Django的form上。 如果我们的规则会被一次又一次的使用，我们可以创建一个自定义的字段类型。 大多数的自定义校验都是一次性的，可以直接绑定到form类.
我们希望`` message`` 字段有一个额外的校验，我们增加一个`` clean_message()`` 方法到`` Form`` 类：
from django import forms
class ContactForm(forms.Form):
subject = forms.CharField(max_length=100)
email = forms.EmailField(required=False)
message = forms.CharField(widget=forms.Textarea)
def clean_message(self):
message = self.cleaned_data['message']
num_words = len(message.split())
if num_words < 4:
raise forms.ValidationError("Not enough words!")
return message
Django的form系统自动寻找匹配的函数方法，该方法名称以clean_开头，并以字段名称结束。 如果有这样的方法，它将在校验时被调用。
特别地，clean_message()方法将在指定字段的默认校验逻辑执行* 之后* 被调用。（本例中，在必填CharField这个校验逻辑之后。）因为字段数据已经被部分处理，所以它被从self.cleaned_data中提取出来了。同样，我们不必担心数据是否为空，因为它已经被校验过了。
我们简单地使用了len()和split()的组合来计算单词的数量。 如果用户输入字数不足，我们抛出一个forms.ValidationError型异常。这个异常的描述会被作为错误列表中的一项显示给用户。
在函数的末尾显式地返回字段的值非常重要。 我们可以在我们自定义的校验方法中修改它的值（或者把它转换成另一种Python类型）。 如果我们忘记了这一步，None值就会返回，原始的数据就丢失掉了。
指定标签
HTML表单中自动生成的标签默认是按照规则生成的：用空格代替下划线，首字母大写。如email的标签是"Email" 。（好像在哪听到过？ 是的，同样的逻辑被用于模块（model）中字段的verbose_name值。 我们在第五章谈到过。）
像在模块中做过的那样，我们同样可以自定义字段的标签。 仅需使用label，像这样：
class ContactForm(forms.Form): subject = forms.CharField(max_length=100) email = forms.EmailField(required=False, **label='Your e-mail address'** ) message = forms.CharField(widget=forms.Textarea)
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