为什么python打印延迟? [英] Why python print is delayed?

问题描述

我正在尝试使用请求下载文件，并在每次检索 100k 大小的文件时打印一个点，但最后打印出所有点.见代码.

I am trying to download file using requests, and print a dot every time retrieve 100k size of file, but all the dots is printed out at the end. See code.

with open(file_name,'wb') as file:
    print("begin downloading, please wait...")
    respond_file = requests.get(file_url,stream=True)
    size = len(respond_file.content)//1000000

    #the next line will not be printed until file is downloaded
    print("the file size is "+ str(size) +"MB")
    for chunk in respond_file.iter_content(102400):
        file.write(chunk)
        #print('',end='.')
        sys.stdout.write('.')
        sys.stdout.flush()
    print("")

推荐答案

您正在此处访问 request.content:

size = len(respond_file.content)//1000000

访问该属性会强制下载整个响应，对于大型响应，这需要一些时间.使用 int(respond_file.headers['content-length']) 代替:

Accessing that property forces the whole response to be downloaded, and for large responses this takes some time. Use int(respond_file.headers['content-length']) instead:

size = int(respond_file.headers['content-length']) // 1000000

Content-Length 标头由服务器提供，由于它是标头的一部分，您无需先下载所有内容即可访问该信息.

The Content-Length header is provided by the server and since it is part of the headers you have access to that information without downloading all of the content first.

如果服务器选择使用 Transfer-Encoding: chunked 来流式传输响应，则无需设置 Content-Length 标头；您可能需要考虑到这一点:

If the server chooses to use Transfer-Encoding: chunked to stream the response, no Content-Length header has to be set; you may need to take that into account:

content_length = respond_file.headers.get('content-length', None)
size_in_kb = '{}KB'.format(int(content_length) // 1024) if content_length else 'Unknown'
print("the file size is", size_in_kb)

其中以千字节为单位的大小是通过将长度除以 1024 来计算的，而不是 100 万.

where the size in kilobytes is calculated by dividing the length by 1024, not 1 million.

或者，在单独的 HEAD 请求中询问大小(仅获取标头):

Alternatively, ask for the size in a separate HEAD request (only fetching the headers):

head_response = requests.get(file_url)
size = int(head_response.headers.get('content-length', 0))

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