如何理解这个表达："future_to_url = {executor.submit(load_url, url, 60): url for url "

Question

当我在官方文档中时，请参阅以下示例：

urls = ['http://www.foxnews.com/',
    'http://www.cnn.com/',
    'http://europe.wsj.com/',
    'http://www.bbc.co.uk/',
    'http://some-made-up-domain.com/']
def load_url(url, timeout):
    with urllib.request.urlopen(url, timeout=timeout) as conn:
        return conn.read()
with concurrent.futures.ThreadPoolExecutor(max_workers=5) as executor:
    future_to_url = {executor.submit(load_url, url, 60): url for url in urls}  
    for future in concurrent.futures.as_completed(future_to_url):
        url = future_to_url[future]
        try:
            data = future.result()
        except Exception as exc:
            print('%r generated an exception: %s' % (url, exc))
        else:
            print('%r page is %d bytes' % (url, len(data)))

但是，我不理解该表达的含义：

"future_to_url = {executor.submit(load_url， url， 60(： url for url ">

语法是从哪里来的吗？谢谢！

Answer 1

它是一个

dict(字典(理解表达式，就像list理解表达式一样。运算符是{}，而不是[]。因为我们用{}定义dictionary文字.例如：

l = [1, 2, 3]
# key is `'name' + str(x)`, value is `x`
print({ 'name' + str(x): x for x in l })
# {'name2': 2, 'name3': 3, 'name1': 1}

上面代码中dict的键是'name' + str(x)，值是x。
所以在{executor.submit(load_url, url, 60): url for url in urls}中，生成的字典的键是executor.submit(load_url, url, 60)，值是url。

有关字典理解

的更多信息，请参阅字典理解。