只打印一次？ [复制]

Question

如何让我的 else 打印只打印一次，而不是为字符串不存在的每一行打印？我尝试通过向后拉几层来移动它，但它不起作用。我理解逻辑，但我不知道如何限制它。我一次添加一点到我的解析脚本中进行练习，边学习边学习，但是这个让我受益匪浅。谢谢！

import csv
# Testing finding something specifical in a CSV, with and else
testpath = 'C:\Users\Devin\Downloads\users.csv'
developer = "devin"

with open (testpath, 'r') as testf:
    testr = csv.reader(testf)
    for row in testr:
        for field in row:
            if developer in row:
                print row
        else:
            print developer +  " does not exist!"

Answer 1

在 Python 中，您可以将 else 子句附加到您的 for 循环。例如

>>> for i in range(10):
...     if i == 5: break # this causes the else statement to be skipped
... else:
...     print 'not found'
...

注意5被发现所以不执行else语句

>>> for i in range(10):
...     if i == 15: break
... else:
...     print 'not found'
...
not found

见documentation on for statements

在第一个套件中执行的 break 语句终止循环 不执行 else 子句的套件。继续声明 在第一个套件中执行会跳过套件的其余部分并继续 使用下一项，如果没有下一项，则使用 else 子句。

Answer 2

先看吉布森的回答。你可以这样做：

for row in testr:
    found = False
    for field in row:
        if developer in row:
            print row
            found = True
            break
    if found: break
else:
    print developer +  " does not exist!"

您也可以省略 found 标志（正如评论中 Jean-François Fabre 所建议的那样），但这使得 imo 有点难以理解（我不得不在脑海中编译）：

for row in testr:       
    for field in row:
        if developer in row:
            print row
            # We found the developer. break from the inner loop.
            break
    else:
        # This means, the inner loop ran fully, developer was not found.
        # But, we have other rows; we need to find more.
        continue
    # This means, the else part of the inner loop did not execute.
    # And that indicates, developer was found. break from the outer loop.
    break
else:
    # The outer loop ran fully and was not broken
    # This means, developer was not found.
    print developer, "does not exist!"