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- Put Most Code Into a Function or Class. Remember that the Python interpreter executes all the code in a module when it imports the module. Sometimes the code you write will have side effects that you want the user to control, such as
- Use if __name__ == "__main__" to Control the Execution of Your Code. What if you want process_data() to execute when you run the script from the command line but not when the Python interpreter imports the file?
- Create a Function Called main() to Contain the Code You Want to Run. Now you are able to write Python code that can be run from the command line as a script and imported without unwanted side effects.
- Call Other Functions From main() Another common practice in Python is to have main() execute other functions, rather than including the task-accomplishing code in main().
Basically, you define __metaclass__ = abc.ABCMeta on the class, then decorate each abstract method with @abc.abstractmethod. Classes derived from this class cannot then be instantiated unless all abstract methods have been overridden.
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Objects have individuality, and multiple names (in multiple scopes) can be bound to the same object. This is known as aliasing in other languages. This is usually not appreciated on a first glance at Python, and can be safely ignored when dealing with immutable basic types (numbers, strings, tuples). However, aliasing has a possibly surprising effe...
A namespace is a mapping from names to objects. Most namespaces are currently implemented as Python dictionaries, but thats normally not noticeable in any way (except for performance), and it may change in the future. Examples of namespaces are: the set of built-in names (containing functions such as abs(), and built-in exception names); the global...
Namespaces are created at different moments and have different lifetimes. The namespace containing the built-in names is 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 state...
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. (Actually, forgetting would be a better way to describe what actually happens.) Of course, recursive invocations each have their own local namespace. The global statement ca...
A scope is a textual region of a Python program where a namespace is directly accessible. Directly accessible here means that an unqualified reference to a name attempts to find the name in the namespace.
Although scopes are determined statically, they are used dynamically. At any time during execution, there are at least three nested scopes whose namespaces are directly accessible:
Data attributes may be referenced by methods as well as by ordinary users (clients) of an object. In other words, classes are not usable to implement pure abstract data types. In fact, nothing in Python makes it possible to enforce data hiding it is all based upon convention. (On the other hand, the Python implementation, written in C, can complet...
Each value is an object, and therefore has a class (also called its type). It is stored as object.__class__. Theres nothing special about instantiation of derived classes: DerivedClassName() creates a new instance of the class. Method references are resolved as follows: the corresponding class attribute is searched, descending down the chain of bas...
Of course, a language feature would not be worthy of the name class without supporting inheritance. The syntax for a derived class definition looks like this: The above example would work even if MappingSubclass were to introduce a __update identifier since it is replaced with _Mapping__update in the Mapping class and _MappingSubclass__update in th...
The name BaseClassName must be defined in a scope containing the derived class definition. In place of a base class name, other arbitrary expressions are also allowed. This can be useful, for example, when the base class is defined in another module:
Derived classes may override methods of their base classes. Because methods have no special privileges when calling other methods of the same object, a method of a base class that calls another method defined in the same base class may end up calling a method of a derived class that overrides it. (For C++ programmers: all methods in Python are effe...
Private instance variables that cannot be accessed except from inside an object dont exist in Python. However, there is a convention that is followed by most Python code: a name prefixed with an underscore (e.g. _spam) should be treated as a non-public part of the API (whether it is a function, a method or a data member). It should be considered an...
Since there is a valid use-case for class-private members (namely to avoid name clashes of names with names defined by subclasses), there is limited support for such a mechanism, called name mangling. Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam, wh...
Note that the mangling rules are designed mostly to avoid accidents; it still is possible to access or modify a variable that is considered private. This can even be useful in special circumstances, such as in the debugger.
Anything that can be done with generators can also be done with class-based iterators as described in the previous section. What makes generators so compact is that the __iter__() and __next__() methods are created automatically.
Another key feature is that the local variables and execution state are automatically saved between calls. This made the function easier to write and much more clear than an approach using instance variables like self.index and self.data.
In addition to automatic method creation and saving program state, when generators terminate, they automatically raise StopIteration. In combination, these features make it easy to create iterators with no more effort than writing a regular function.
Some simple generators can be coded succinctly as expressions using a syntax similar to list comprehensions but with parentheses instead of square brackets. These expressions are designed for situations where the generator is used right away by an enclosing function. Generator expressions are more compact but less versatile than full generator defi...
A function inside a class and associated with an object or class is called a Method. Similar to functions, methods also have a name, parameters, and a return statement. Classes can bundle data and functionality together. We use methods to provide the functionality to a class.
May 25, 2022 · In this guide, I’ll explain how including a main() function, though not required in Python, can structure your programs in a logical way and ensure that your functions are executed only when expected.
Sep 27, 2021 · Many programming languages have the notion of a main function (or a main method), which acts as the entry point for a program. Python does not have main functions. Python runs all code in Python scripts. We have a Python script called greet.py:
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1 day ago · Built-in Functions ¶. The Python interpreter has a number of functions and types built into it that are always available. They are listed here in alphabetical order. abs(x) ¶. Return the absolute value of a number. The argument may be an integer, a floating-point number, or an object implementing __abs__().
