实现 DISTAL 应用程序#

Implementing the DISTAL application

中文

现在我们已经获得了数据，可以开始实现 DISTAL 应用程序了。为了简化操作，我们将使用 CGI 脚本来实现用户界面。

备注

CGI 脚本并不是实现 DISTAL 应用程序的唯一方法。其他可能的方法包括使用 Web 应用框架，如 TurboGears 或 Django，使用 AJAX 编写自己的动态 Web 应用，使用 CherryPy（http://cherrypy.org），甚至使用像 Pyjamas（http://pyjs.org）这样的工具将 Python 代码编译成 JavaScript。然而，这些方法比 CGI 更复杂，我们将在本章中使用 CGI 脚本，以便尽可能简化代码。

让我们来看一下我们的 CGI 脚本将如何实现 DISTAL 应用程序的工作流：

如你所见，有三个独立的 CGI 脚本，分别是 selectCountry.py、selectArea.py 和 showResults.py，每个脚本实现了 DISTAL 应用程序的不同部分。

小技巧

什么是 CGI 脚本？

虽然编写 CGI 脚本的细节超出了本书的范围，但基本概念是将原始 HTML 输出打印到标准输出，并使用内置的 cgi 模块处理来自浏览器的 CGI 参数。

要将 Python 程序作为 CGI 脚本运行，你需要做两件事：首先，在脚本的开头添加一个 “shebang” 行，像这样：

#!/usr/bin/python

对于 MS Windows，添加以下行：

#!C:Python27python.exe -U

你使用的确切路径取决于你在计算机上安装 Python 的位置。

第二件事是，至少在类 Unix 系统上，需要将脚本设置为可执行。例如：

chmod +x selectCountry.py

更多信息，请参见网络上常见的 CGI 教程，例如：[http://wiki.python.org/moin/CgiScripts](http://wiki.python.org/moin/CgiScripts)。

让我们从创建一个简单的 Web 服务器开始，它能够运行我们的 CGI 脚本。对于 Python 来说，这很简单；只需创建以下程序，我们称之为 webServer.py：

import BaseHTTPServer
import CGIHTTPServer

address = ('', 8000)
handler = CGIHTTPServer.CGIHTTPRequestHandler
server = BaseHTTPServer.HTTPServer(address, handler)
server.serve_forever()

接下来，在与 webServer.py 程序相同的目录下创建一个名为 cgi-bin 的子目录。这个子目录将保存你创建的各种 CGI 脚本。

运行 webServer.py 将会在 http://127.0.0.1:8000 设置一个 Web 服务器，执行你放入 cgi-bin 子目录中的任何 CGI 脚本。例如，要访问 selectCountry.py 脚本，你可以在 Web 浏览器中输入以下 URL：

http://127.0.0.1:8000/cgi-bin/selectCountry.py

英文

Now that we have the data, we can start to implement the DISTAL application itself. To keep things simple, we will use CGI scripts to implement the user interface.

备注

CGI scripts aren’t the only way we could implement the DISTAL application. Other possible approaches include using web application frameworks such as TurboGears or Django, using AJAX to write your own dynamic web application, using CherryPy (http://cherrypy.org) or even using tools such as Pyjamas (http://pyjs. org) to compile Python code into JavaScript. All of these approaches, however, are more complicated than CGI, and we will be making use of CGI scripts in this chapter to keep the code as straightforward as possible.

Let’s take a look at how our CGI scripts will implement the DISTAL application’s workflow:

As you can see, there are three separate CGI scripts, selectCountry.py, selectArea.py, and showResults.py, each implementing a distinct part of the DISTAL application.

小技巧

What is a CGI Script?

While the details of writing CGI scripts are beyond the scope of this book, the basic concept is to print the raw HTML output to stdout, and to process the incoming CGI parameters from the browser using the built-in cgi module.

To run a Python program as a CGI script, you have to do two things: first, you have to add a “shebang” line to the start of the script, like this:

#!/usr/bin/python

For MS Windows, add the following line:

#!C:Python27python.exe -U

The exact path you use will depend on where you have Python installed on your computer.

The second thing you need to do, at least on Unix-like systems, is make your script executable. For example:

chmod +x selectCountry.py

For more information, see one of the CGI tutorials commonly available on the Internet, for example: http://wiki.python.org/moin/ CgiScripts.

Let’s start by creating a simple web server capable of running our CGI scripts. With Python this is easy; simply create the following program, which we will call webServer.py:

import BaseHTTPServer
import CGIHTTPServer

address = ('', 8000)
handler = CGIHTTPServer.CGIHTTPRequestHandler
server = BaseHTTPServer.HTTPServer(address, handler)
server.serve_forever()

Next, create a subdirectory named cgi-bin within the same directory as your webServer.py program. This subdirectory will hold the various CGI scripts you create.

Running webServer.py will set up a web server at http://127.0.0.1:8000, which will execute any CGI scripts you place into the cgi-bin subdirectory. So, for example, to access the selectCountry.py script, you would enter the following URL into your web browser:

http://127.0.0.1:8000/cgi-bin/selectCountry.py

共享“数据库”模块#

The shared “database” module

中文

为了简化操作，我们将把所有与数据库相关的代码放入一个单独的模块中，我们称之为 database.py。下面是该模块的基本结构，并实现了 database.open() 函数，我们将在 CGI 脚本中使用该函数来打开数据库连接：

# database.py

import os.path
import pyproj
from shapely.geometry import Polygon
import shapely.wkt

##############################################################
# 根据需要编辑这些常量，以匹配你的设置。

MYSQL_DBNAME   = "distal"
MYSQL_USERNAME = "XXX"
MYSQL_PASSWORD = "XXX"

POSTGIS_DBNAME = "distal"
POSTGIS_USERNAME = "XXX"
POSTGIS_PASSWORD = "XXX"

SPATIALITE_DB_PATH = os.path.join(os.path.dirname(__file__),
                                    "..", "distal.db")

DB_TYPE = "XXX"

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

def open():
    global _connection, _cursor

    if DB_TYPE == "MySQL":
        import MySQLdb
        _connection = MySQLdb.connect(user=MYSQL_USERNAME,
                                        passwd=MYSQL_PASSWORD)
        _cursor = _connection.cursor()
        _cursor.execute("USE "+MYSQL_DBNAME)
    elif DB_TYPE == "PostGIS":
        import psycopg2

        params = []
        params.append("dbname=" + POSTGIS_DBNAME)
        if POSTGIS_USERNAME != None:
            params.append("user=" + POSTGIS_USERNAME)
        if POSTGIS_PASSWORD != None:
            params.append("password=" + POSTGIS_PASSWORD)
        _connection = psycopg2.connect(" ".join(params))
        _cursor = _connection.cursor()
    elif DB_TYPE == "SpatiaLite":
        from pysqlite2 import dbapi2 as sqlite
        _connection = sqlite.connect(SPATIALITE_DBNAME)
        _connection.enable_load_extension(True)
        _connection.execute('SELECT load_extension("...")')
        _cursor = _connection.cursor()
    else:
        raise RuntimeError("Unknown database type: " +
                            db_type)

确保将此 database.py 模块放置在与 CGI 脚本相同的目录中。

不要忘记编辑模块顶部的常量，以匹配你特定的设置，输入适当的数据库名称、用户名和密码等。

备注

SPATIALITE_DB_PATH 常量设置为我们 distal.db 文件的绝对路径。我们使用 Python 内置的 __file__ 全局变量，避免将路径硬编码到模块中。

请注意，我们使用私有的全局变量（以单个下划线字符为前缀）来存储数据库连接和游标。这使得我们可以在以后访问这些变量，随着我们向模块添加更多功能。

英文

To make things easier, we’ll put all the database-specific code into a separate module, which we’ll call database.py. Here is the basic structure for this module, along with the implementation of the database.open() function, which we’ll use in our CGI scripts to open a connection to the database:

# database.py

import os.path
import pyproj
from shapely.geometry import Polygon
import shapely.wkt

##############################################################
# Edit these constants as necessary to match your setup.

MYSQL_DBNAME   = "distal"
MYSQL_USERNAME = "XXX"
MYSQL_PASSWORD = "XXX"

POSTGIS_DBNAME = "distal"
POSTGIS_USERNAME = "XXX"
POSTGIS_PASSWORD = "XXX"

SPATIALITE_DB_PATH = os.path.join(os.path.dirname(__file__),
                                  "..", "distal.db")

DB_TYPE = "XXX"

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

def open():
    global _connection, _cursor

    if DB_TYPE == "MySQL":
        import MySQLdb
        _connection = MySQLdb.connect(user=MYSQL_USERNAME,
                                      passwd=MYSQL_PASSWORD)
        _cursor = _connection.cursor()
        _cursor.execute("USE "+MYSQL_DBNAME)
    elif DB_TYPE == "PostGIS":
        import psycopg2

        params = []
        params.append("dbname=" + POSTGIS_DBNAME)
        if POSTGIS_USERNAME != None:
            params.append("user=" + POSTGIS_USERNAME)
        if POSTGIS_PASSWORD != None:
            params.append("password=" + POSTGIS_PASSWORD)
        _connection = psycopg2.connect(" ".join(params))
        _cursor = _connection.cursor()
    elif DB_TYPE == "SpatiaLite":
        from pysqlite2 import dbapi2 as sqlite
        _connection = sqlite.connect(SPATIALITE_DBNAME)
        _connection.enable_load_extension(True)
        _connection.execute('SELECT load_extension("...")')
        _cursor = _connection.cursor()
    else:
        raise RuntimeError("Unknown database type: " +
                           db_type)

Make sure you place this database.py module into the same directory as your CGI scripts.

Don’t forget to edit the constants at the top of the module to match your particular setup, entering the appropriate database names, usernames and passwords, and so on.

备注

The SPATIALITE_DB_PATH constant is set to the absolute path to our distal.db file. We use the Python’s built-in __file__ global to avoid having to hardwire paths into our module.

Note that we use private global variables (prefixed with an underscore character) to store the database connection and cursor. This lets us access these variables later on, as we add more functions to this module.

“选择国家”脚本#

The “select country” script

中文

selectCountry.py 脚本的任务是向用户显示一个国家列表，以便用户选择一个所需的国家，然后将所选国家传递给 selectArea.py 脚本以进行进一步处理。

以下是 selectCountry.py 脚本输出的样子：

这个 CGI 脚本非常基础：我们只需要打印出 HTML 页面内容，让用户从一个国家名称列表中选择一个国家：

#!/usr/bin/python

import database
database.open()

print 'Content-Type: text/html; charset=UTF-8\n\n'
print '<html>'
print '<head><title>Select Country</title></head>'
print '<body>'
print '<form method="POST" action="selectArea.py">'
print '<select name="countryID" size="10">'

for id,name in database.list_countries():
    print '<option value="'+str(id)+'">'+name+'</option>'

print '</select>'
print '<p>'
print '<input type="submit" value="OK">'
print '</form>'
print '</body>'
print '</html>'

小技巧

理解 HTML 表单

如果你之前没有使用过 HTML 表单，别担心。它们非常简单，如果你愿意，你可以直接复制这里给出的示例代码。如果想了解更多关于 HTML 表单的信息，可以查阅网上的许多教程。一个不错的例子可以在 http://www.pagetutor.com/form_tutor 找到。

如你所见，我们在 database 模块中调用了 list_countries() 函数，返回了一个国家记录的 ID 和关联的名称列表。该函数的实现很简单；只需在 database.py 模块中添加以下代码：

def list_countries():
    global _cursor
    results = []
    _cursor.execute("SELECT id,name FROM countries " +
                    "ORDER BY name")
    for id,name in _cursor:
        results.append((id, name))
    return results

不幸的是，这段代码有一个问题：由于 SpatiaLite 无法处理 UTF-8 字符编码，我们必须在返回之前手动将国家名称从 Unicode 转换为 UTF-8。我们可以通过在函数中添加以下高亮部分来完成此操作：

这完成了“选择国家”脚本的编写。现在，你应该能够通过在浏览器中输入以下 URL 来运行它：

http://127.0.0.1:8000/cgi-bin/selectCountry.py

如果一切顺利，你应该能看到国家列表，并且能够选择一个国家。如果你点击 OK 按钮，你应该会看到一个 404 错误，表示 selectArea.py 脚本尚未存在——这完全正确，因为我们还没有实现它。

英文

The task of the selectCountry.py script is to display a list of countries to the user, so that the user can choose a desired country which is then passed to the selectArea.py script for further processing.

Here is what the selectCountry.py script’s output will look like:

This CGI script is very basic: we simply print out the contents of the HTML page which lets the user choose a country from a list of country names:

#!/usr/bin/python

import database
database.open()

print 'Content-Type: text/html; charset=UTF-8\n\n'
print '<html>'
print '<head><title>Select Country</title></head>'
print '<body>'
print '<form method="POST" action="selectArea.py">'
print '<select name="countryID" size="10">'

for id,name in database.list_countries():
    print '<option value="'+str(id)+'">'+name+'</option>'

print '</select>'
print '<p>'
print '<input type="submit" value="OK">'
print '</form>'
print '</body>'
print '</html>'

小技巧

Understanding HTML Forms

If you haven’t used HTML forms before, don’t panic. They are quite straightforward, and if you want you can just copy the code from the examples given here. To learn more about HTML forms, check out one of the many tutorials available online. A good example can be found at http://www.pagetutor.com/form_tutor.

As you can see, we call the list_countries() function within the database module to return a list of country record IDs and their associated names. The implementation of this function is straightforward; simply add the following code to your database. py module:

def list_countries():
    global _cursor
    results = []
    _cursor.execute("SELECT id,name FROM countries " +
                    "ORDER BY name")
    for id,name in _cursor:
        results.append((id, name))
    return results

Unfortunately, there is a problem with this code: because SpatiaLite can’t handle UTF-8 character encoding, we have to manually convert the country name from Unicode to UTF-8 before returning it. We can do this by adding the following highlighted lines to our function:

This completes the “select country” script. You should now be able to run it by typing the following URL in your web browser:

http://127.0.0.1:8000/cgi-bin/selectCountry.py

All going well, you should see the list of countries and be able to select one. If you click on the OK button, you should see a 404 error, indicating that the selectArea. py script doesn’t exist yet—which is perfectly correct, as we haven’t implemented it yet.

实现 DISTAL 应用程序#

共享“数据库”模块#

“选择国家”脚本#

“选择区域”脚本#

计算边界框#

计算地图尺寸#

设置数据源#

渲染地图图像#

“显示结果”脚本#

识别点击点#

按距离识别特征#

手动计算距离#

使用角距离#

使用投影坐标#

混合方法#

显示结果#