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# 11.1 作为客户端与HTTP服务交互

## 问题

你需要通过 HTTP 协议以客户端的方式访问多种服务。例如，下载数据或者与基于 REST 的 API 进行交互。

## 解决方案

对于简单的事情来说，通常使用 `urllib.request` 模块就够了。例如，发送一个简单的 HTTP GET 请求到远程的服务上，可以这样做：

```python
from urllib import request, parse

# Base URL being accessed
url = 'http://httpbin.org/get'

# Dictionary of query parameters (if any)
parms = {
   'name1' : 'value1',
   'name2' : 'value2'
}

# Encode the query string
querystring = parse.urlencode(parms)

# Make a GET request and read the response
u = request.urlopen(url+'?' + querystring)
resp = u.read()
```

如果你需要使用 POST 方法在请求主体中发送查询参数，可以将参数编码后作为可选参数提供给 `urlopen()` 函数，就像这样：

```python
from urllib import request, parse

# Base URL being accessed
url = 'http://httpbin.org/post'

# Dictionary of query parameters (if any)
parms = {
   'name1' : 'value1',
   'name2' : 'value2'
}

# Encode the query string
querystring = parse.urlencode(parms)

# Make a POST request and read the response
u = request.urlopen(url, querystring.encode('ascii'))
resp = u.read()
```

如果你需要在发出的请求中提供一些自定义的 HTTP 头，例如修改 `user-agent` 字段,可以创建一个包含字段值的字典，并创建一个 Request 实例然后将其传给 `urlopen()` ，如下：

```python
from urllib import request, parse
...

# Extra headers
headers = {
    'User-agent' : 'none/ofyourbusiness',
    'Spam' : 'Eggs'
}

req = request.Request(url, querystring.encode('ascii'), headers=headers)

# Make a request and read the response
u = request.urlopen(req)
resp = u.read()
```

关于 requests 库，一个值得一提的特性就是它能以多种方式从请求中返回响应结果的内容。从上面的代码来看， `resp.text` 带给我们的是以 Unicode 解码的响应文本。但是，如果去访问 `resp.content` ，就会得到原始的二进制数据。另一方面，如果访问 `resp.json` ，那么就会得到 JSON 格式的响应内容。

下面这个示例利用 `requests` 库发起一个 HEAD 请求，并从响应中提取出一些 HTTP 头数据的字段：

```python
import requests

resp = requests.head('http://www.python.org/index.html')

status = resp.status_code
last_modified = resp.headers['last-modified']
content_type = resp.headers['content-type']
content_length = resp.headers['content-length']
```

下面是一个利用 requests 通过基本认证登录 Pypi 的例子：

```python
import requests

resp = requests.get('http://pypi.python.org/pypi?:action=login',
                    auth=('user','password'))
```

下面是一个利用 requests 将 HTTP cookies 从一个请求传递到另一个的例子：

```python
import requests

# First request
resp1 = requests.get(url)
...

# Second requests with cookies received on first requests
resp2 = requests.get(url, cookies=resp1.cookies)
```

用 requests 上传内容：

```python
import requests
url = 'http://httpbin.org/post'
files = { 'file': ('data.csv', open('data.csv', 'rb')) }

r = requests.post(url, files=files)
```

## 讨论

如果你决定坚持使用标准的程序库而不考虑像 `requests` 这样的第三方库，那么也许就不得不使用底层的 `http.client` 模块来实现自己的代码。比方说，下面的代码展示了如何执行一个 HEAD 请求：

```python
from http.client import HTTPConnection
from urllib import parse

c = HTTPConnection('www.python.org', 80)
c.request('HEAD', '/index.html')
resp = c.getresponse()

print('Status', resp.status)
for name, value in resp.getheaders():
    print(name, value)
```

下面这个示例实现在 Python 包索引上的认证：

```python
import urllib.request

auth = urllib.request.HTTPBasicAuthHandler()
auth.add_password('pypi','http://pypi.python.org','username','password')
opener = urllib.request.build_opener(auth)

r = urllib.request.Request('http://pypi.python.org/pypi?:action=login')
u = opener.open(r)
resp = u.read()

# From here. You can access more pages using opener
...
```

在开发过程中测试 HTTP 客户端代码常常是很令人沮丧的，因为所有棘手的细节问题都需要考虑（例如 cookies、认证、HTTP 头、编码方式等）。要完成这些任务，考虑使用 `httpbin` 服务（[http://httpbin.org](http://httpbin.org/)）。这个站点会接收发出的请求，然后以 JSON 的形式将相应信息回传回来。下面是一个交互式的例子：

```python
>>> import requests
>>> r = requests.get('http://httpbin.org/get?name=Dave&n=37',
...     headers = { 'User-agent': 'goaway/1.0' })
>>> resp = r.json
>>> resp['headers']
{'User-Agent': 'goaway/1.0', 'Content-Length': '', 'Content-Type': '',
'Accept-Encoding': 'gzip, deflate, compress', 'Connection':
'keep-alive', 'Host': 'httpbin.org', 'Accept': '*/*'}
>>> resp['args']
{'name': 'Dave', 'n': '37'}
```

在要同一个真正的站点进行交互前，先在 `httpbin.org` 这样的网站上做实验常常是可取的办法。

`request` 库还对许多高级的 HTTP 客户端协议提供了支持，比如 `OAuth`。


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