About Python

About Python

• 1.
  Shao-Chuan Wang Presentation version 2.5.4
• 2.
   Python Objectand Types  Object and Class Model  Name Scope Object Life Cycle  Exception Handling
• 3.
  Everything is anobject!
• 4.
   New-style objectv.s. Old-Style Objects  The following presentation mainly focuses on new-style objects.
• 5.
   Type checkingat compile-time V.S. run-time ◦ Statically typed Dynamically typed int a a  Explicit V.S. Implicit type conversion ◦ Strongly-typed v.s. weakly-typed  Python is dynamically and strongly typed
• 6.
   <type ‘object’>is an instance of <type ‘type’>  <type ‘type’> is an instance of itself  <type ‘type’> is a subclass/subtype of <type ‘object’> <type ‘type’> type of all types <type ‘object’> bases of all other types
• 7.
  • New atype object by subtyping • New an object by explicit instantiation • The type object serves as a factory that can churn out new objects (to each of which it is related as type-instance). This is why every object has a type. <type ‘type’> type of all types <type ‘object’> bases of all other types <class ‘C’>
• 8.
  • Generate a‘class’ at runtime. • Generate a ‘function’ at runtime. • Very flexible and generic
• 9.
   They areobjects.  Not All (but most of) type objects are able to create instance objects via the static method __new__. ◦ type(typeObj) -> <type ‘type’> ◦ callable(typeObj) -> True  Not all (but most of) type object are subclassable (e.g. FunctionType)
• 10.
  Enhancing productivity andreadability
• 11.
  • All Pythonobjects have: – an identity (an integer, returned by id(x)) – a type (returned by type(x)) – some content/value • Once the object is created, you cannot change the type nor the identity. You may or may not change the content of the object. – Mutable object: objects whose content/value can change – Immutable object: objects whose content/value is unchangeable once they are created • ‘string’ is an object of type <type ‘str’>, and has a unique id.
• 12.
   Names referto objects; names are the ‘keys’ to obtain the objects. (a little bit similar to reference type in C)  An object can have many names, or no name at all.  Names live in namespaces (introduced in the next slide)  They are not the properties of the object
• 13.
  • Namespaces arecollections of (name, object reference) pairs (implemented using dictionaries). • globals(): the namespace of the __main__ module • The built-in namespace associated with the execution of a code block is actually found by looking up the name __builtins__ in its global namespace • Every object can have its namespace. (e.g. a module namespace, an instance namespace, a function’s local namespace)
• 14.
   An attributeis a way to get from one object to another, by applying the dot operation ◦ objectName.attrName -> anotherObject ◦ objectName.attrName  objectName.__getattribute__(‘attrName')  The object and anotherObject can be of any kind, including module, class, function/method, and all other kinds of instance objects.  The vague hierarchy of ‘class’ and ‘object’. Class B can include object a, and object a now can also include Class B.
• 15.
  • The objectmust be created or already exist before assignment • objectName.attrName = anotherObj  objectName.__setattr__(‘attrName’, anotherObj) • What does an assignment really do? – Binding the name, and the object – Adding a new attribute to the ‘local’ namespace
• 16.
  • Assignment oneobject to another object is like copying its reference to the name. (in terms of C language) • It actually re-bind the name and the object, so the object now has several names Python: C/C++: a: 0 at 0013FEE0 b: 5 at 0013FEDC a: 5 at 0013FEE0 b: 5 at 0013FEDC # before a = b {‘a’:0, ‘b’: 5} # after a = b {‘a’:5, ‘b’: 5}
• 17.
  • Names are‘references’ (implemented by PyObject * in C) to objects. • Assignment a literal integer object (the value larger) to a name is to allocate a new memory containing that value and return the reference. Python: C/C++: 0013FEE0 0013FEE0
• 18.
   Creating aninstance of a Class by calling __new__(cls, …)  There is no way to create a new instance without calling __new__  cls. __new__() is a static method
• 19.
   Descriptor definition:An object with a __get__() method, and optionally __set__() and __delete__() methods, accepting specific parameters is said to follow the descriptor protocol. 1. d.__get__(cobj, C) 2. d.__set__(cobj, 5) 3. d.__delete__(cobj)
• 20.
   Data Descriptor: ◦A descriptor with both __get__() and __set__() methods  Non-data Descriptor ◦ Descriptors with only the __get__() method. - Can be hidden by Instance attribute.
• 21.
   A functionis actually a non-data descriptor.  An easier way to generate a descriptor: property
• 22.
  • Class functionand instance method are actually different objects • cobj.f => C.f.__get__(cobj, C)
• 23.
  • cobj.d getsthe return value of __get__, implemented in the descriptor class. • C.__dict__[‘d’] gets the descriptor object. • They are of course different (in general).
• 24.
   The ‘local’namespace (mapping of ‘name’ and ‘object reference’) is actually saved in __dict__
• 25.
  • obj.attrName ->invoking obj.__getattribute__(‘attrName’) – If attrName is a Python-provided attribute, return it. – Search obj.__class__.__dict__ for attrName . If it exists and is a data-descriptor, return the descriptor result. Search all bases of obj.__class__ for the same case. – Search obj.__dict__ for attrName , and return if found. If obj is a type object, search its bases too. If it is a type object and a descriptor is found in the object or its bases, return the descriptor result. – Search obj.__class__.__dict__ for attrName . If it exists and is a non-data descriptor, return the descriptor result. If it exists, and is not a descriptor, just return it. Search all bases of objectname.__class__ for same case. – Raise AttributeError
• 26.
   What ifMulti-Inheritance? c.a -> ??? d.a -> ??? Why?
• 27.
  • Searching attributesin the order of __mro__ • __mro__ is a data descriptor of <type ‘type’> • mro: Method Resolution Order
• 28.
   A morecomplex example e.a -> ??? Be careful about multi-inheritance and naming conflict
• 29.
  • If Bis a subclass of A, then B should always appear before A in all __mro__s. Let’s denote B > A • If C appears before D in the list of bases in a class statement (e.g. Z(C, D)), then C > D MRO computing principle: • If E > F in one scenario (or one __mro__), then it should be that E > F in all scenarios (or all __mro__s). A: X > Y, B: Y > X, so when C(A, B) => Conflict
• 30.
   L[E] =E + Merge(DBAO, CAO, DC) = E + D + Merge(BA, CAO, C) = E + D + B + Merge(A,CAO,C) = E + D + B + C + Merge(A,AO) = E + D + B + C + A + O Object B A D C E
• 32.
  • Call byreference? •Why?
• 33.
   Assignment isto re-bind the name and object in the ‘local’ namespace. In the case, the object names in the function are saved in func_code.co_varnames, and func_code.co_argcount.  Assignment cannot change the content of the object.
• 34.
  • Calling methodof the argument object in the function:
• 35.
  1 2 4 1 2 2 1 2 3 or ??? 1 2 3
• 36.
  1 2 1 error when invoking!!
• 37.
  3 5 1 6
• 38.
  • When returningand passing the function. Be careful that the name conflict may cause the problem. • Binding objects in func_closure when used object in the enclosing area. • Scopes are determined statically
• 39.
  • Built-in namespaceis created when the Python interpreter starts up, and is never deleted. • The global namespace for a module is created when the module definition is read in; normally, module namespaces also last until the interpreter quits • The local namespace for a function is created when the function is called, and deleted when the function returns or raises an exception that is not handled within the function. recursive invocations each have their own local namespace
• 40.
  • import moduleName:the module object comes into the global dictionary. • from moduleName import someObject: the some object comes into the global dictionary.
• 41.
   Encapsulation ◦ Publicobject access, but read-only by default! ◦ Outer inner scope cannot change the inner outer scope object’s value if programmer does not provide the interfaces methods. ◦ The only way to change the content value of the object is to through the object itself.
• 43.
  File write completed successfully Iffile exists, and open successfully Unexpected error
• 44.
  • “Catching StandardError”or “Unable to open file”?
• 45.
   Exception typicallyis a class object (of type <type ‘type’>) IOError EnvironmentError TypeError Exception BaseException object StandardError LookupError KeyError Exception Inheritance Tree StopIterationSystemExit
• 46.
   All built-inexception objects are defined in exceptions module
• 47.
   When anexception is not handled at all, the interpreter terminates execution of the program, or returns to its interactive main loop
• 48.
   Let callerto handle the exception
• 50.
   raise ‘string’,‘value’  raise ExCObject, ‘value’  raise ExCObject(‘value’) exInst = ExCObject(‘value’)  raise exInst  raise exInstObj
• 51.
   In pastversions of Python string exceptions were supported. In Python 1.5 and newer versions, all standard exceptions have been converted to class objects and users are encouraged to do the same. String exceptions will raise a DeprecationWarning in Python 2.5 and newer. In future versions, support for string exceptions will be removed.
• 52.
   We canactually catch Exception instance, and handle it.  Take IOError for example, it has args, errno, filename, message, strerror attributes, they are all data descriptors  args defined in BaseException  errno, filename, message, strerror defined in EnvironmentError
• 54.
  • Inherit ‘Exception’class object: • repr() returns the canonical string representation of the object. • __str__() is called by the str built-in function and by the print statement to compute the informal string representation of an object
• 55.
   What isthe output?
• 56.
  • L[C] =C + D + EOF + IO + ENV + S S EOF Env D IO C Avoid Multi-inheritance From different Exception Type Objects
• 57.
  • The problemwith this code is that it leaves the file open for an indeterminate amount of time after the code has finished executing. This is not an issue in simple scripts, but can be a problem for larger applications. • After the statement is executed, the file f is always closed, even if a problem was encountered while processing the lines.
• 58.
  • The abovecode roughly translates into this: • var.__exit__() records predefined clean-up actions http://www.python.org/dev/peps/pep-0343/
• 59.
  Other tricks aboutPython
• 60.
   Difference betweena & b ?  a save in A.__dict__, and b save in instanceOfA.__dict__  To rebind a’s value in A, only by Class A object, cannot through any instances of A.
• 61.
  • List comprehension –s = [x ** 2 for x in range(10)] – v = [2 ** x for x in range(10)] – zeros = [0 for I in range(10)] • Iterator & Generator – g = (x * 2 for x in range(1000)) => g.next(), g.next(), g.next() – a function which can stop whatever it is doing at an arbitrary point in its body, return a value back to the caller, and, later on, resume from the point it had `frozen'
• 62.
   ‘yield’ statement
• 63.
  • Loop reada file: without occupying too much memory.
• 64.
   Generator application:Permutation generator function: return a generator implicitly v = s.next() => call/resume the generator function call boundary condition
• 65.
  Thank you!