Pythonic 打破循环的方法

Question

这里是python初学者。

我有以下函数，它检查从某些输入派生的字符串是否存在于文本文件中。它循环遍历文本文件的每一行，以查看是否找到了完全匹配的内容。

我必须在找到匹配项后立即跳出循环，以避免不必要的循环。

代码如下：

def DateZoneCity_downloaded_previously(Order_Date,ZoneCity):    #   function to check if a given DateZoneCity
                                                                    #   combination had already been completely downloaded
    string_to_match = Order_Date.strftime('%Y/%m/%d') + "-" + ZoneCity[0] + "-" + ZoneCity[1]
    with open(Record_File) as download_status:
        DateZoneCity_exists = False
        for line in download_status:
            if string_to_match in line:
                DateZoneCity_exists = True                      #   if match found, then set "DateZoneCity_exists" to True
                break                                           #   and break out from the [for line in download_status:] loop
        if DateZoneCity_exists: return True
    download_status.close()

我正在寻找一种更简洁、pythonic 的方式来构建代码。我能做些什么来让这变得更好吗？以某种方式消除对“DateZoneCity_exists”和第二个 If 语句的需要？

Answer 1

这感觉就像any 是最好的解决方案：

# Function to check if a given DateZoneCity
def DateZoneCity_downloaded_previously(Order_Date, ZoneCity):
    # Combination had already been completely downloaded
    string_to_match = Order_Date.strftime('%Y/%m/%d') + "-" + ZoneCity[0]
                                                      + "-" + ZoneCity[1]
    with open(Record_File) as download_status:
        return any((string_to_match in line) for line in download_status)

请注意，在这种情况下，它将返回负数 False，而不是返回 None 的当前实现，另请注意，它会在找到正数结果后立即跳出循环，因此不需要循环以任何一种方式遍历整个文件。

Answer 2

只需return 而不是break：

def DateZoneCity_downloaded_previously(Order_Date,ZoneCity):
    """Check if a given DataZoneCity combination had already been downloaded."""
    string_to_match = Order_Date.strftime('%Y/%m/%d') + "-" + ZoneCity[0] + "-" + ZoneCity[1]
    with open(Record_File) as download_status:
        for line in download_status:
            if string_to_match in line:
                return True
    return False  # No match found.

Answer 3

根据你的文本文件有多大，你可以将它读入一个字符串，然后直接使用它（比每行读取和检查行更容易而且通常更快）：

if string_to_match in open(Record_File).read():
    return True

在你的例子中：

def DateZoneCity_downloaded_previously(Order_Date,ZoneCity):                   
    string_to_match = Order_Date.strftime('%Y/%m/%d') + "-" + ZoneCity[0] + "-" + ZoneCity[1]
    if string_to_match in open(Record_File).read():
        return True

Answer 4

你不必在此处放置 for 循环，请参阅更新后的代码。

def DateZoneCity_downloaded_previously(Order_Date,ZoneCity):
    """Check if a given DataZoneCity combination had already been downloaded."""
    string_to_match = Order_Date.strftime('%Y/%m/%d') + "-" + ZoneCity[0] + "-" + ZoneCity[1]
    with open(Record_File) as download_status:
        if string_to_match in download_status.read():
            return True
    return False  # No match found.

Answer 5

如果record_file 很小，那么你可以直接使用in 和ifstatement，比如：-

def DateZoneCity_downloaded_previously(Order_Date,ZoneCity):                   
    string_to_match = Order_Date.strftime('%Y/%m/%d') + "-" + ZoneCity[0] + "-" + ZoneCity[1]
    if string_to_match in open(Record_File).read():
        return True  

如果 record_file 很大，那么您必须遍历每一行以找到匹配项，例如：-

def DateZoneCity_downloaded_previously(Order_Date, ZoneCity):
    string_to_match = Order_Date.strftime('%Y/%m/%d') + "-" + ZoneCity[0]  + "-" + ZoneCity[1]
    with open(Record_File) as download_status:
        return is_empty((string_to_match in line) for line in download_status)  

因为逐行读取会节省用于保存行的存储空间。