Python 中类的前向声明
以下程序可以成功运行：
class Simple(object):
def __init__(self, name):
self.name = name
def __add__(self, other):
c = Composite()
c._members.append(self)
c._members.append(other)
return c
def __repr__(self):
return "Simple('%s')" % self.name
class Composite(object):
def __init__(self):
self._members = []
def __add__(self, obj):
if isinstance(obj, Simple):
out = Composite()
out._members = [k for k in self._members] + [obj]
elif isinstance(obj, Composite):
out = Composite()
out._members = [k for k in self._members + obj._members]
else:
raise TypeError
return out
if __name__ == "__main__":
s1 = Simple('one')
s2 = Simple('two')
s3 = Simple('three')
c1 = s1 + s2
c2 = c1 + s3
print c1._members
print c2._members
# output:
# [Simple('one'), Simple('two')]
# [Simple('one'), Simple('two'), Simple('three')]
我想将 Simple、Composite 和 __main__ 的定义保留在三个不同的文件中，但我不能这样做是因为我无法将 Simple 导入到 composite.py 中，并且无法将 Composite 导入到 simple.py 中。
您将如何修改类定义以便可以保留单独的文件？
谢谢。
附言。我已经阅读了几个与“前向声明”相关的答案，但找不到我的具体问题的答案。
The following program can run successfully:
class Simple(object):
def __init__(self, name):
self.name = name
def __add__(self, other):
c = Composite()
c._members.append(self)
c._members.append(other)
return c
def __repr__(self):
return "Simple('%s')" % self.name
class Composite(object):
def __init__(self):
self._members = []
def __add__(self, obj):
if isinstance(obj, Simple):
out = Composite()
out._members = [k for k in self._members] + [obj]
elif isinstance(obj, Composite):
out = Composite()
out._members = [k for k in self._members + obj._members]
else:
raise TypeError
return out
if __name__ == "__main__":
s1 = Simple('one')
s2 = Simple('two')
s3 = Simple('three')
c1 = s1 + s2
c2 = c1 + s3
print c1._members
print c2._members
# output:
# [Simple('one'), Simple('two')]
# [Simple('one'), Simple('two'), Simple('three')]
I would like to keep the definition of Simple, Composite, and __main__ in three different files, but I cannot do it because I cannot import Simple into composite.py and I cannot import Composite into simple.py.
How would you modify the class definition such that you can keep separate files?
Thank you.
PS. I've read several answers related to "forward declaration" but could not find an answer to my specific problem.
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由于在调用方法之前不需要任何引用，因此循环导入可以在这里工作。诀窍是使用完全限定的引用。
Since none of the references are needed until the methods are called, circular imports can work here. The trick is to use fully-qualified references.
问题是你得到了循环导入，对吧？
import Simple 在
__add__方法中而不是在文件顶部，以避免循环依赖。它会稍微减慢 __add__ 方法的速度，但通常不会很明显。The problem is that you get circular imports, right?
import Simple in the
__add__method instead of at the top of the file to avoid circular dependency. It will slow down the__add__method somewhat, but usually not significantly.