Python 字符串相等测试结果不一致

Question

下面这个函数是在一个脚本中用来创建一个Django网站的静态版本：

def write_file(filename, content):
    filename = '{0}{1}.html'.format(BASEDIR, filename)
    if os.path.exists(filename):
        existing_file = io.open(filename, encoding='utf-8')
        existing_content = existing_file.read()
        existing_file.close()
        if existing_content != content:
            print "Content is not equal, writing file to {0}".format(filename)
            encoded_content = content.encode('utf-8')
            html_file = open(filename, 'w')
            html_file.write(encoded_content)
            html_file.close()
        else:
            print "Content is equal, nothing is written to {0}".format(filename)

当我运行这个脚本两次（数据库没有任何变化），正常情况下应该不会有任何写入操作。但是奇怪的是，有超过一半的文件被反复写入。

Answer 1

你描述的情况很可能是数据在某个环节被编码了两次，或者是文本在和unicode进行比较。在Python 2.x中，abc` == u`abc，所以一些只包含ASCII字符的文件在比较时会通过测试，而另一部分包含非ASCII字符的文件在经过UTF-8编码前后就不会相同了。

要弄清楚发生了什么，最简单的方法就是改善你代码中的错误报告：在else语句之后，添加：

print repr(existing_content), repr(content)

Answer 2

我建议使用 codecs 这个模块；可以这样做：

import codecs

def write_file(filename, content):
    filename = "{0}{1}.html".format(BASEDIR, filename)   
    if os.path.exists(filename):

        # open file and read into a utf8 string.
        # Calling open(), read(), then close() can all be made into 1 call.
        # python will handle the closing and gc for you
        existing_content = codecs.open(filename, "r", "utf-8").read()

        if existing_content != content.encode("utf-8"):
            print "Content is not equal, writing file to {0}".format(filename)
            
            # python will close the open fd for you after this
            # codecs will handle the utf8 conversion before writing to the file, so no need to encode 'content'
            codecs.open(filename, "w", "utf-8").write(content)

            # Although, it might be necessary to write the utf-8 Byte-Order Marker first:
            outF = open(filename, "w")
            outF.write(codecs.BOM_UTF8)
            outF.write(content.encode("utf-8"))
            outF.close()
        else:
            print "Content is equal, nothing is written to {0}".format(filename)

这里有很多有用的信息：如何在Python中使用utf-8