Python 多行帶語句

Question

什么是干凈的方法來創建一個多行with Python中？ 我想在一個with打開多個文件，但它的右邊足夠遠，我希望它在多行上。 像這樣：

class Dummy:
    def __enter__(self): pass
    def __exit__(self, type, value, traceback): pass

with Dummy() as a, Dummy() as b,
     Dummy() as c:
    pass

不幸的是，這是一個SyntaxError 。 所以我試過這個：

with (Dummy() as a, Dummy() as b,
      Dummy() as c):
    pass

也是語法錯誤。 但是，這有效：

with Dummy() as a, Dummy() as b,\
     Dummy() as c:
    pass

但是如果我想發表評論怎么辦？ 這不起作用：

with Dummy() as a, Dummy() as b,\
     # my comment explaining why I wanted Dummy() as c\
     Dummy() as c:
    pass

\\ s 的位置也沒有任何明顯的變化。

有沒有干凈的方式來創建一個多行with陳述，讓里面的意見？

Answer 1

鑒於您已經標記了這個 Python 3，如果您需要用上下文管理器散布注釋，我會使用contextlib.ExitStack ：

from contextlib import ExitStack

with ExitStack() as stack:
    a = stack.enter_context(Dummy()) # Relevant comment
    b = stack.enter_context(Dummy()) # Comment about b
    c = stack.enter_context(Dummy()) # Further information

這相當於

with Dummy() as a, Dummy() as b, Dummy() as c:

這樣做的好處是您可以循環生成上下文管理器，而無需單獨列出每個管理器。 文檔給出了一個例子，如果你想打開一堆文件，並且你有一個列表中的文件名，你可以這樣做

with ExitStack() as stack:
    files = [stack.enter_context(open(fname)) for fname in filenames]

如果您的上下文管理器占用了太多的屏幕空間，以至於您想在它們之間放置評論，那么您可能有足夠的空間想要使用某種循環。

---

正如 Deathless 先生在評論中提到的那樣，PyPI 上有一個名為contextlib2 contextlib反向移植。 如果您使用的是 Python 2，則可以使用ExitStack的 backport 實現。

---

順便說一句，你不能做這樣的事情的原因

with (
        ThingA() as a,
        ThingB() as b):
    ...

是因為 a (也可以是上下文管理器的表達式的第一個標記，並且 CPython 的當前解析器在看到第一個(時無法判斷它應該解析什么規則。這是一個激勵示例對於PEP 617 ，它引入了一個更強大的新解析器，因此您想要的語法可能很快就會出現。

Answer 2

這對我來說似乎最整潔：

with open('firstfile', 'r') as (f1 # first
  ), open('secondfile', 'r') as (f2 # second
  ):
    pass

Answer 3

僅限 Python 3.9+：

with (
    Dummy() as a,
    Dummy() as b,
    # my comment explaining why I wanted Dummy() as c
    Dummy() as c,
):
    pass

蟒≤3.8：

with \
    Dummy() as a, \
    Dummy() as b, \
    Dummy() as c:
    pass

不幸的是，這種語法無法進行注釋。

---

這是它有效的證據：

Python 3.9.0a6 (default, Jun 20 2020, 14:52:53) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.15.0 -- An enhanced Interactive Python. Type '?' for help.

In [3]: with (open('x') 
   ...:        as f): 
   ...:     pass 
   ...:                                                                                                                                                                                                 
---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-3-47d5a51db08b> in <module>
----> 1 with (open('x')
      2        as f):
      3     pass
      4 

FileNotFoundError: [Errno 2] No such file or directory: 'x'

In [4]:                                                                                                                                                                                                 
Do you really want to exit ([y]/n)? y

wPython 3.8.2 (default, May  8 2020, 20:08:31) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.15.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: with (open('x') 
   ...:       as f): 
   ...:     pass                                                                                                                                                                                        
  File "<ipython-input-1-e538abd13934>", line 2
    as f):
    ^
SyntaxError: invalid syntax

Answer 4

這並不完全干凈，但你可以這樣做：

with Dummy() as a, Dummy() as b, (
     #my comment
     Dummy()) as c:
    pass

沒有語法錯誤，但它不是最干凈的。 你也可以這樣做：

with Dummy() as a, Dummy() as b, Dummy(
     #my comment
     ) as c:
    pass

考慮找到一種不使用with中間的注釋的方法。

Answer 5

我會通過在with語句之前或在行本身添加注釋來使事情變得簡單易讀：

# my comment explaining why I wanted Dummy() as c
with Dummy() as a, Dummy() as b,\
     Dummy() as c: # or add the comment here
    pass

Answer 6

像TigerhawkT3's answer ，但縮進不會觸發pycodestyle 的錯誤 E124 ：

with (
        open('firstfile', 'r')) as f1, (  # first
        open('secondfile', 'r')) as f2:  # second
    pass

IMO它仍然很丑，但至少它通過了短絨。