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An example of a simple implementation of a proxy class

Goal: to implement the instance properties of class Product to allow another class Proxy to proxy access and control. You want to give the published attributes to the proxy class for external access and control. Attributes that do not want to be exposed cannot be accessed and controlled through the agent. Here, the attribute conventions that do not want to be published start with an underscore.

# proxy_example1.py# the following is an example of a proxy class that implements read-only access # goal: only public properties of Product can be accessed and modified after a proxy Private attribute _ current can only be viewed and cannot be modified class Product: def _ _ init__ (self, price, quantity): self.price = price self.quantity = quantity self._current = 12 only expose proxy class Proxy for external use class Proxy: def _ init__ (self, obj): self._obj = obj def _ getattr__ (self Item): # attributes not found in this instance will execute the _ _ getattr__ method if item.startswith ("_"): # it is stipulated that the method at the beginning of the underscore cannot access the proxied class Only access to the private properties of the proxy class raise Exception (f "{item} not found") # Product and do not want to be known to the outside world return getattr (self._obj, item) def _ setattr__ (self, key, value): if key.startswith ("_"): # it is stipulated that the method at the beginning of the underscore cannot access the proxied class Only the proxy class will be accessed # Note: raise is not allowed here This will cause instances of Proxy not to be created (attributes such as _ _ dict__ cannot be created) super (Proxy, self). _ _ setattr__ (key, value) # avoid infinite loops else: setattr (self._obj, key, value) # requires that only attributes def _ _ delattr__ (self) that are not beginning with an underscore can be deleted Item): if item.startswith ("_"): super (Proxy, self). _ _ delattr__ (item) # avoid infinite loops else: delattr (self._obj, item) def test_getattr (): P = Product (10,1) pp = Proxy (p) print (pp.price) print (pp._curr) def test_setattr (): P = Product (10) 2) pp = Proxy (p) pp.abc = 1 print (pp.abc, p.abc) pp._curr = 10000 print (pp._curr) # Private attribute Set to proxy class print (p._curr) # raise an error, def test_delattr (): P = Product (10,2) pp = Proxy (p) pp.abc = 123 print (pp.abc, p.abc) # Delete public attribute del pp.abc # successful # print (pp.abc) P.abc) # has been deleted # # Delete the private attribute # del pp._curr # will try to delete the private attribute of Proxy Raise AttributeError: _ curr # first create the instance property print (pp._def) of Proxy after deleting pp._def = 123#. The proxied Product instance does not create _ def property # del pp._def # deletes the instance property of Proxy # print (pp._def)

Test get properties

If _ _ name__ = ='_ _ main__': test_getattr ()

Output:

ten

...

Exception: _ curr not found

...

Test Settings Properties

If _ _ name__ = ='_ _ main__': test_delattr ()

Output

1 1

10000

...

AttributeError: 'Product' object has no attribute' _ curr'

...

Test delete properties

If _ _ name__ = ='_ _ main__': test_delattr ()

Output

123 123

one hundred and twenty three

Note: a method that begins and ends with a double underscore cannot be proxied. If you want to use it, you must define this method in the proxy class, and then redirect it to the method of the proxied class. For example, if you want to use the isinstance () method, you have to forge the definition of _ _ class__ attribute in Proxy, and if you want to use the len () method, you have to redirect to the len method of the proxied class in Proxy.

# proxy_example2.pyclass Product: def _ init__ (self, price, quantity): self.price = price self.quantity = quantity self._current = 123def _ len__ (self): return 11 expose only proxy Proxy for external use class Proxy: def _ init__ (self, obj): self._obj = obj def _ getattr__ (self) Item): # attributes not found in this instance will execute the _ _ getattr__ method if item.startswith ("_"): # it is stipulated that the method at the beginning of the underscore cannot access the proxied class Only access to the private properties of the proxy class raise Exception (f "{item} not found") # Product and do not want to be known to the outside world return getattr (self._obj, item) def _ setattr__ (self, key, value): if key.startswith ("_"): # it is stipulated that the method at the beginning of the underscore cannot access the proxied class Only the proxy class will be accessed # Note: raise is not allowed here This will cause instances of Proxy not to be created (attributes such as _ _ dict__ cannot be created) super (Proxy, self). _ _ setattr__ (key, value) # avoid infinite loops else: setattr (self._obj, key, value) # requires that only attributes def _ _ delattr__ (self) that are not beginning with an underscore can be deleted Item): if item.startswith ("_"): super (Proxy, self). _ _ delattr__ (item) # avoid infinite loops else: delattr (self._obj Item) @ property def _ class__ (self): # forgery class return self._obj.__class__ def _ _ len__ (self): return len (self._obj) def test_instance (): P = Product (10,2) pp = Proxy (p) print (pp.__class__) print (isinstance (pp) Product)) # if you don't forge _ _ class__ Return False def test_len (): P = Product (10,2) pp = Proxy (p) print (len (pp)) # if the Proxy instance does not define a _ _ len__ method, it will report an error TypeError: object of type 'Proxy' has no len ()

Test forged instance class type

If _ _ name__ = ='_ _ main__': test_instance ()

Output

True

Test acquisition length

If _ _ name__ = ='_ _ main__': test_len ()

Output

one hundred and eleven

A simplified example of a proxy class that implements log output

An example of capturing web server error logs and performing exception handling

# logger_proxy.py#-*-coding:utf-8-*-from functools import wrapsclass DAL: @ classmethod def dm1 (cls, req, * args): print ("dm1...", f "{req=}") print (1 req= 0) # deliberately throws an exception return "dm1" class BLL: @ classmethod def bm1 (cls, req): print ("bm1...") F "{req=}") return DAL.dm1 (req) class Application: def _ _ init__ (self, req): self.req = req self._p = "private attr" def hd1 (self): return BLL.bm1 (self.req) class LoggerProxy: def _ init__ (self, obj): self._obj = obj def _ getattr__ (self) Item): # attributes not obtained by the instance of the LoggerProxy class will execute this method attr = getattr (self._obj, item) if callable (attr): # get the method Handle exception catch @ wraps (attr) def wrapped_method (* args, * * kwargs): # print (f "Before access to attribute/method: {item}") try: method = attr (* args, * * kwargs) except ZeroDivisionError: # catch exception and then handle. Raise Exception (f "{attr.__name__} received a zero division error.") # print (f "After attribute/method {item} returned") return method return wrapped_method else: # got the attribute Directly return return attrif _ _ name__ = ='_ main__': lp = LoggerProxy (Application ("abc")) print (lp.req) print (lp._p) print (lp.hd1 ())

Running output

Abc

Private attr

Bm1... Req='abc'

Dm1... Req='abc'

Traceback...

ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback...

Exception: hd1 received a zero division error.

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