如何理解這個表達式： 「future_to_url = {executor.submit（load_url，網址，60）：URL對於URL」

Question

當我在官方文檔，see this example：

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))) 

不過，我不噸understant表達的含義是：

「future_to_url = {executor.submit（load_url，網址，60）：URL對於URL」

是它從該語法點？謝謝！

Answer 1

這是一個dict（字典）理解表達，就像list理解表達。運營商是{}，而不是[]。因爲我們定義dictionary文字與{}。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} 

的字典的在上面的代碼的關鍵是'name' + str(x)，該值是x。
所以在{executor.submit(load_url, url, 60): url for url in urls}，生成字典的關鍵是executor.submit(load_url, url, 60)，值是url。

關於字典理解的更多信息，請參閱Dict Comprehensions。