Конструктор в питоне это

Обновлено: 15.06.2024

This chapter describes some things you’ve learned about already in more detail, and adds some new things as well.

5.1. More on Lists¶

The list data type has some more methods. Here are all of the methods of list objects:

Add an item to the end of the list. Equivalent to a[len(a):] = [x] .

list. extend ( iterable )

Extend the list by appending all the items from the iterable. Equivalent to a[len(a):] = iterable .

Insert an item at a given position. The first argument is the index of the element before which to insert, so a.insert(0, x) inserts at the front of the list, and a.insert(len(a), x) is equivalent to a.append(x) .

Remove the first item from the list whose value is equal to x. It raises a ValueError if there is no such item.

Remove the item at the given position in the list, and return it. If no index is specified, a.pop() removes and returns the last item in the list. (The square brackets around the i in the method signature denote that the parameter is optional, not that you should type square brackets at that position. You will see this notation frequently in the Python Library Reference.)

Remove all items from the list. Equivalent to del a[:] .

Return zero-based index in the list of the first item whose value is equal to x. Raises a ValueError if there is no such item.

The optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument.

Return the number of times x appears in the list.

list. sort ( * , key = None , reverse = False )

Sort the items of the list in place (the arguments can be used for sort customization, see sorted() for their explanation).

Reverse the elements of the list in place.

Return a shallow copy of the list. Equivalent to a[:] .

An example that uses most of the list methods:

You might have noticed that methods like insert , remove or sort that only modify the list have no return value printed – they return the default None . 1 This is a design principle for all mutable data structures in Python.

5.1.1. Using Lists as Stacks¶

The list methods make it very easy to use a list as a stack, where the last element added is the first element retrieved (“last-in, first-out”). To add an item to the top of the stack, use append() . To retrieve an item from the top of the stack, use pop() without an explicit index. For example:

5.1.2. Using Lists as Queues¶

It is also possible to use a list as a queue, where the first element added is the first element retrieved (“first-in, first-out”); however, lists are not efficient for this purpose. While appends and pops from the end of list are fast, doing inserts or pops from the beginning of a list is slow (because all of the other elements have to be shifted by one).

To implement a queue, use collections.deque which was designed to have fast appends and pops from both ends. For example:

5.1.3. List Comprehensions¶

List comprehensions provide a concise way to create lists. Common applications are to make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.

For example, assume we want to create a list of squares, like:

Note that this creates (or overwrites) a variable named x that still exists after the loop completes. We can calculate the list of squares without any side effects using:

which is more concise and readable.

A list comprehension consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses. The result will be a new list resulting from evaluating the expression in the context of the for and if clauses which follow it. For example, this listcomp combines the elements of two lists if they are not equal:

and it’s equivalent to:

Note how the order of the for and if statements is the same in both these snippets.

If the expression is a tuple (e.g. the (x, y) in the previous example), it must be parenthesized.

List comprehensions can contain complex expressions and nested functions:

5.1.4. Nested List Comprehensions¶

The initial expression in a list comprehension can be any arbitrary expression, including another list comprehension.

Consider the following example of a 3x4 matrix implemented as a list of 3 lists of length 4:

The following list comprehension will transpose rows and columns:

As we saw in the previous section, the nested listcomp is evaluated in the context of the for that follows it, so this example is equivalent to:

which, in turn, is the same as:

In the real world, you should prefer built-in functions to complex flow statements. The zip() function would do a great job for this use case:

See Unpacking Argument Lists for details on the asterisk in this line.

5.2. The del statement¶

There is a way to remove an item from a list given its index instead of its value: the del statement. This differs from the pop() method which returns a value. The del statement can also be used to remove slices from a list or clear the entire list (which we did earlier by assignment of an empty list to the slice). For example:

del can also be used to delete entire variables:

Referencing the name a hereafter is an error (at least until another value is assigned to it). We’ll find other uses for del later.

5.3. Tuples and Sequences¶

We saw that lists and strings have many common properties, such as indexing and slicing operations. They are two examples of sequence data types (see Sequence Types — list, tuple, range ). Since Python is an evolving language, other sequence data types may be added. There is also another standard sequence data type: the tuple.

A tuple consists of a number of values separated by commas, for instance:

As you see, on output tuples are always enclosed in parentheses, so that nested tuples are interpreted correctly; they may be input with or without surrounding parentheses, although often parentheses are necessary anyway (if the tuple is part of a larger expression). It is not possible to assign to the individual items of a tuple, however it is possible to create tuples which contain mutable objects, such as lists.

Though tuples may seem similar to lists, they are often used in different situations and for different purposes. Tuples are immutable , and usually contain a heterogeneous sequence of elements that are accessed via unpacking (see later in this section) or indexing (or even by attribute in the case of namedtuples ). Lists are mutable , and their elements are usually homogeneous and are accessed by iterating over the list.

A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective. For example:

The statement t = 12345, 54321, 'hello!' is an example of tuple packing: the values 12345 , 54321 and 'hello!' are packed together in a tuple. The reverse operation is also possible:

This is called, appropriately enough, sequence unpacking and works for any sequence on the right-hand side. Sequence unpacking requires that there are as many variables on the left side of the equals sign as there are elements in the sequence. Note that multiple assignment is really just a combination of tuple packing and sequence unpacking.

5.4. Sets¶

Python also includes a data type for sets. A set is an unordered collection with no duplicate elements. Basic uses include membership testing and eliminating duplicate entries. Set objects also support mathematical operations like union, intersection, difference, and symmetric difference.

Curly braces or the set() function can be used to create sets. Note: to create an empty set you have to use set() , not <> ; the latter creates an empty dictionary, a data structure that we discuss in the next section.

Here is a brief demonstration:

Similarly to list comprehensions , set comprehensions are also supported:

5.5. Dictionaries¶

Another useful data type built into Python is the dictionary (see Mapping Types — dict ). Dictionaries are sometimes found in other languages as “associative memories” or “associative arrays”. Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by keys, which can be any immutable type; strings and numbers can always be keys. Tuples can be used as keys if they contain only strings, numbers, or tuples; if a tuple contains any mutable object either directly or indirectly, it cannot be used as a key. You can’t use lists as keys, since lists can be modified in place using index assignments, slice assignments, or methods like append() and extend() .

It is best to think of a dictionary as a set of key: value pairs, with the requirement that the keys are unique (within one dictionary). A pair of braces creates an empty dictionary: <> . Placing a comma-separated list of key:value pairs within the braces adds initial key:value pairs to the dictionary; this is also the way dictionaries are written on output.

The main operations on a dictionary are storing a value with some key and extracting the value given the key. It is also possible to delete a key:value pair with del . If you store using a key that is already in use, the old value associated with that key is forgotten. It is an error to extract a value using a non-existent key.

Performing list(d) on a dictionary returns a list of all the keys used in the dictionary, in insertion order (if you want it sorted, just use sorted(d) instead). To check whether a single key is in the dictionary, use the in keyword.

Here is a small example using a dictionary:

The dict() constructor builds dictionaries directly from sequences of key-value pairs:

In addition, dict comprehensions can be used to create dictionaries from arbitrary key and value expressions:

When the keys are simple strings, it is sometimes easier to specify pairs using keyword arguments:

5.6. Looping Techniques¶

When looping through dictionaries, the key and corresponding value can be retrieved at the same time using the items() method.

When looping through a sequence, the position index and corresponding value can be retrieved at the same time using the enumerate() function.

To loop over two or more sequences at the same time, the entries can be paired with the zip() function.

To loop over a sequence in reverse, first specify the sequence in a forward direction and then call the reversed() function.

To loop over a sequence in sorted order, use the sorted() function which returns a new sorted list while leaving the source unaltered.

Using set() on a sequence eliminates duplicate elements. The use of sorted() in combination with set() over a sequence is an idiomatic way to loop over unique elements of the sequence in sorted order.

It is sometimes tempting to change a list while you are looping over it; however, it is often simpler and safer to create a new list instead.

5.7. More on Conditions¶

The conditions used in while and if statements can contain any operators, not just comparisons.

The comparison operators in and not in are membership tests that determine whether a value is in (or not in) a container. The operators is and is not compare whether two objects are really the same object. All comparison operators have the same priority, which is lower than that of all numerical operators.

Comparisons may be combined using the Boolean operators and and or , and the outcome of a comparison (or of any other Boolean expression) may be negated with not . These have lower priorities than comparison operators; between them, not has the highest priority and or the lowest, so that A and not B or C is equivalent to (A and (not B)) or C . As always, parentheses can be used to express the desired composition.

The Boolean operators and and or are so-called short-circuit operators: their arguments are evaluated from left to right, and evaluation stops as soon as the outcome is determined. For example, if A and C are true but B is false, A and B and C does not evaluate the expression C . When used as a general value and not as a Boolean, the return value of a short-circuit operator is the last evaluated argument.

It is possible to assign the result of a comparison or other Boolean expression to a variable. For example,

Note that in Python, unlike C, assignment inside expressions must be done explicitly with the walrus operator := . This avoids a common class of problems encountered in C programs: typing = in an expression when == was intended.

5.8. Comparing Sequences and Other Types¶

Sequence objects typically may be compared to other objects with the same sequence type. The comparison uses lexicographical ordering: first the first two items are compared, and if they differ this determines the outcome of the comparison; if they are equal, the next two items are compared, and so on, until either sequence is exhausted. If two items to be compared are themselves sequences of the same type, the lexicographical comparison is carried out recursively. If all items of two sequences compare equal, the sequences are considered equal. If one sequence is an initial sub-sequence of the other, the shorter sequence is the smaller (lesser) one. Lexicographical ordering for strings uses the Unicode code point number to order individual characters. Some examples of comparisons between sequences of the same type:

Note that comparing objects of different types with < or >is legal provided that the objects have appropriate comparison methods. For example, mixed numeric types are compared according to their numeric value, so 0 equals 0.0, etc. Otherwise, rather than providing an arbitrary ordering, the interpreter will raise a TypeError exception.

Other languages may return the mutated object, which allows method chaining, such as d->insert("a")->remove("b")->sort(); .

Конструктор в Python – это особый тип метода (функции), который используется для инициализации членов экземпляра класса.

В C ++ или Java конструктор имеет то же имя, что и его класс, в Python конструктор обрабатывается по-разному. Он используется для создания объекта.

Конструкторы бывают двух типов:

  1. Параметризованный конструктор
  2. Непараметрический конструктор

Определение конструктора выполняется, когда мы создаем объект этого класса. Конструкторы также проверяют, что у объекта достаточно ресурсов для выполнения любой задачи запуска.

Создание конструктора на Python

В Python метод __init __() имитирует конструктор класса. Этот метод вызывается при создании экземпляра класса. Он принимает ключевое слово self в качестве первого аргумента, который позволяет получить доступ к атрибутам или методу класса.

Мы можем передать любое количество аргументов во время создания объекта класса, в зависимости от определения __init __(). В основном он используется для инициализации атрибутов класса. У каждого класса должен быть конструктор, даже если он просто полагается на конструктор по умолчанию.

Рассмотрим следующий пример для инициализации атрибутов класса Employee при работе с конструкторами в Python.

Подсчет количества объектов класса

Конструктор вызывается автоматически, когда мы создаем объект класса. Рассмотрим следующий пример.

Непараметрический

Непараметрический конструктор используется, когда мы не хотим манипулировать значением, или конструктором, который имеет только self в качестве аргумента. Разберем на примере.

Параметризованный конструктор Python

У параметризованного конструктора есть несколько параметров вместе с самим собой.

Конструктор Python по умолчанию

Когда мы не включаем конструктор в класс или забываем его объявить, он становится конструктором по умолчанию. Он не выполняет никаких задач, а инициализирует объекты. Рассмотрим пример.

Более одного конструктора в одном классе

Давайте посмотрим на другой сценарий, что произойдет, если мы объявим два одинаковых конструктора в классе.

В приведенном выше коде объект st вызвал второй конструктор, тогда как оба имеют одинаковую конфигурацию. Первый метод недоступен для объекта st. Внутренне объект класса всегда будет вызывать последний конструктор, если у класса есть несколько конструкторов.

Примечание. Перегрузка конструктора в Python запрещена.

Встроенные функции классов Python

Встроенные функции, определенные в классе, описаны в следующей таблице.

SN Функция Описание
1 getattr(obj,name,default) Используется для доступа к атрибуту объекта.
2 setattr(obj, name,value) Она используется для установки определенного значения для определенного атрибута объекта.
3 delattr (obj, name) Необходима для удаления определенного атрибута.
4 hasattr (obj, name) Возвращает истину, если объект содержит определенный атрибут.

Встроенные атрибуты класса

Наряду с другими атрибутами класс Python также содержит некоторые встроенные атрибуты класса, которые предоставляют информацию о классе.

Объектно-ориентированный шаблон в Python является наиболее распространенным типом шаблона проектирования, и его можно найти почти в каждом языке программирования высокого уровня. В этом методе мы создаем объект и конкретный класс и используем функциональность класса, используя объект этого определенного класса. Понятия oops состоят из следующих компонентов:

  • Класс
  • Объект
  • Конструктор
  • Наследование
  • Инкапсуляция
  • Полиморфизм

Мы кратко обсудим вышеуказанные компоненты.

Класс

Класс — это план объекта; он содержит методы и конструкторы. Мы используем ключевое слово class для определения класса. Класс Python будет выглядеть следующим образом.

Класс может содержать специальные атрибуты, начинающиеся с двойного подчеркивания(__). Например, __init__() — непараметризованный конструктор.

Объект

Объект используется для доступа к атрибутам класса. Мы можем создать объект класса следующим образом.

Конструкторы

Конструкторы — это специальные методы, которые автоматически вызываются всякий раз, когда создается экземпляр нового объекта этого класса. Они начинаются с двойного подчеркивания __ и называются специальными функциями. Конструктор определяется как функция __int__(). Он используется для инициализации всех переменных.

Наследование

Наследование — это процесс передачи свойства одного класса другому классу. Класс, производный от свойства другого класса, называется подклассом или дочерним классом, а класс, производный от другого класса, называется родительским классом или базовым классом. Наследование обеспечивает возможность повторного использования кода.

Давайте разберем следующий пример.

Собственный параметр

Self работает в Python так же, как «this» в C++. Он относится к текущему классу функции, и мы можем получить доступ к данным в функции класса. Он работает как переменная, но не вызывает данные.

Инкапсуляция

В отличие от C++, Python не поддерживает такие ключевые слова, как public, protected и private, для определения доступности данных. По умолчанию все данные в Python общедоступны. Но он предоставляет возможность определить любой метод или переменную private. Мы можем использовать «__», чтобы скрыть его свойство перед переменной или методом. Давайте разберем следующий пример:

Существует специальный синтаксис, который можно использовать для доступа к закрытым данным или методу в Python. Синтаксис на самом деле является его именем изменения. Давайте разберем следующий пример.

В приведенном выше коде мы определили переменную age как закрытый член класса. Для доступа к нему мы использовали специальный синтаксис с использованием объекта класса./p>

Полиморфизм

Полиморфизм — один из наиболее важных аспектов концепции oops. Python — это язык с динамической типизацией, который отслеживает типы переменных. Полиморфизм используется для достижения многих целей одним и тем же способом.

Как видно из приведенного выше кода, операция «добавить» поддерживается не только целым числом, но и строкой. Это простой пример полиморфизма.

Многие операции и функции в Python полиморфны. Итак, пока мы используем полиморфные операторы и функции, полиморфизм будет существовать, даже если у нас нет этой цели.

Преимущества объектно-ориентированного шаблона

Ниже приведены несколько важных преимуществ шаблона проектирования Oops:

  • Он поддерживает привязку динамического типа, что означает, что экземпляр класса может динамически изменяться во время выполнения.
  • Python поддерживает множественное наследование и перегрузку операторов с расширенным определением, которого нет в некоторых языках ООП.
  • Концепция oops в Python не поддерживает ключевое слово private, public и protected, поскольку это усложняет синтаксис. Как сказал изобретатель Python Гвидо Ван Россум: «Изобилие синтаксиса приносит больше хлопот, чем помощи». Мы можем повторно использовать код, используя наследование.
  • Полиморфизм обеспечивает гибкость программы. Используя полиморфизм, мы можем создать такой же метод в родительском классе, и он будет работать во всех дочерних классах.
  • Обеспечивает эффективное решение проблем. Концепция ООП позволяет нам разбивать код на небольшие задачи, которые мы затем можем решить.

Недостатки объектно-ориентированного шаблона

Шаблон Python ООП также имеет некоторые недостатки. Эти недостатки приведены ниже.

Как и другие языки программирования общего назначения, Python с самого начала является объектно-ориентированным языком. Это позволяет нам разрабатывать приложения с использованием объектно-ориентированного подхода. В Python мы можем легко создавать и использовать классы и объекты.

Объектно-ориентированная парадигма заключается в разработке программы с использованием классов и объектов. Объект связан с сущностями реального слова, такими как книга, дом, карандаш и т. д. Концепция Oops в Python фокусируется на написании повторно используемого кода. Это широко распространенный метод решения проблемы путем создания объектов.

Ниже приведены основные принципы системы объектно-ориентированного программирования:

    ;
  • Метод; ;
  • Полиморфизм; ;
  • Инкапсуляция.

Класс

Класс можно определить как набор объектов. Это логическая сущность, имеющая определенные атрибуты и методы. Например: если у вас есть класс сотрудника, он должен содержать атрибут и метод, то есть идентификатор электронной почты, имя, возраст, зарплату и т. д.

Объект

Объект – это сущность, у которой есть состояние и поведение. Это может быть любой объект реального мира, например, мышь, клавиатура, стул, стол, ручка и т. д.

В Python все является объектом, и почти все имеет атрибуты и методы. Все функции имеют встроенный атрибут __doc__, который возвращает строку документации, определенную в исходном коде функции.

Когда мы определяем класс, он должен создать объект для выделения памяти. Рассмотрим следующий пример.

В приведенном выше примере мы создали класс с именем car, и у него есть два атрибута modelname и year. Мы создали объект c1 для доступа к атрибуту класса. Объект c1 выделит память для этих значений. Мы узнаем больше о классе и объекте в следующем уроке.

Метод

Метод – это функция, связанная с объектом. В Python метод не уникален для экземпляров класса. У любого типа объекта могут быть методы.

Наследование

Наследование – это наиболее важный аспект объектно-ориентированного программирования, который имитирует реальную концепцию наследования. Он указывает, что дочерний объект приобретает все свойства и поведение родительского объекта.

Используя наследование, мы можем создать класс, который использует все свойства и поведение другого класса. Новый класс известен как производный класс или дочерний класс, а тот, свойства которого получены, известен как базовый класс или родительский класс.

Это обеспечивает возможность повторного использования кода.

Полиморфизм

Полиморфизм состоит из двух слов «poly» (много) и «morphs» (форма). Под полиморфизмом мы понимаем, что одну задачу можно выполнять по-разному. Например – у вас класс животных, и все животные говорят. Но “говорят” по-иному. Здесь «говорящее» поведение в некотором смысле полиморфно и зависит от животного. Итак, абстрактное понятие «животное» на самом деле не «говорит», но конкретные животные (например, собаки и кошки) имеют конкретную реализацию действия «говорить».

Инкапсуляция

Инкапсуляция также является важным аспектом объектно-ориентированного программирования. Она используется для ограничения доступа к методам и переменным. При инкапсуляции код и данные объединяются в один блок от случайного изменения.

Абстракция данных

И абстракция данных, и инкапсуляция часто используются как синонимы. Являются почти однозначными, поскольку абстракция данных достигается за счет инкапсуляции.

Абстракция используется, чтобы скрыть внутренние детали и показать только функциональные возможности. Абстрагирование чего-либо означает присвоение имен вещам, чтобы имя отражало суть того, что делает функция или вся программа.

Объектно-ориентированные и процедурно-ориентированные языки программирования

Разница между объектно-ориентированным и процедурно-ориентированным программированием приведена ниже:

Classes provide a means of bundling data and functionality together. Creating a new class creates a new type of object, allowing new instances of that type to be made. Each class instance can have attributes attached to it for maintaining its state. Class instances can also have methods (defined by its class) for modifying its state.

Compared with other programming languages, Python’s class mechanism adds classes with a minimum of new syntax and semantics. It is a mixture of the class mechanisms found in C++ and Modula-3. Python classes provide all the standard features of Object Oriented Programming: the class inheritance mechanism allows multiple base classes, a derived class can override any methods of its base class or classes, and a method can call the method of a base class with the same name. Objects can contain arbitrary amounts and kinds of data. As is true for modules, classes partake of the dynamic nature of Python: they are created at runtime, and can be modified further after creation.

In C++ terminology, normally class members (including the data members) are public (except see below Private Variables ), and all member functions are virtual. As in Modula-3, there are no shorthands for referencing the object’s members from its methods: the method function is declared with an explicit first argument representing the object, which is provided implicitly by the call. As in Smalltalk, classes themselves are objects. This provides semantics for importing and renaming. Unlike C++ and Modula-3, built-in types can be used as base classes for extension by the user. Also, like in C++, most built-in operators with special syntax (arithmetic operators, subscripting etc.) can be redefined for class instances.

(Lacking universally accepted terminology to talk about classes, I will make occasional use of Smalltalk and C++ terms. I would use Modula-3 terms, since its object-oriented semantics are closer to those of Python than C++, but I expect that few readers have heard of it.)

9.1. A Word About Names and Objects¶

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 effect on the semantics of Python code involving mutable objects such as lists, dictionaries, and most other types. This is usually used to the benefit of the program, since aliases behave like pointers in some respects. For example, passing an object is cheap since only a pointer is passed by the implementation; and if a function modifies an object passed as an argument, the caller will see the change — this eliminates the need for two different argument passing mechanisms as in Pascal.

9.2. Python Scopes and Namespaces¶

Before introducing classes, I first have to tell you something about Python’s scope rules. Class definitions play some neat tricks with namespaces, and you need to know how scopes and namespaces work to fully understand what’s going on. Incidentally, knowledge about this subject is useful for any advanced Python programmer.

Let’s begin with some definitions.

By the way, I use the word attribute for any name following a dot — for example, in the expression z.real , real is an attribute of the object z . Strictly speaking, references to names in modules are attribute references: in the expression modname.funcname , modname is a module object and funcname is an attribute of it. In this case there happens to be a straightforward mapping between the module’s attributes and the global names defined in the module: they share the same namespace! 1

Attributes may be read-only or writable. In the latter case, assignment to attributes is possible. Module attributes are writable: you can write modname.the_answer = 42 . Writable attributes may also be deleted with the del statement. For example, del modname.the_answer will remove the attribute the_answer from the object named by modname .

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 statements executed by the top-level invocation of the interpreter, either read from a script file or interactively, are considered part of a module called __main__ , so they have their own global namespace. (The built-in names actually also live in a module; this is called builtins .)

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.

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 3 or 4 nested scopes whose namespaces are directly accessible:

the innermost scope, which is searched first, contains the local names

the scopes of any enclosing functions, which are searched starting with the nearest enclosing scope, contains non-local, but also non-global names

the next-to-last scope contains the current module’s global names

the outermost scope (searched last) is the namespace containing built-in names

If a name is declared global, then all references and assignments go directly to the middle scope containing the module’s global names. To rebind variables found outside of the innermost scope, the nonlocal statement can be used; if not declared nonlocal, those variables are read-only (an attempt to write to such a variable will simply create a new local variable in the innermost scope, leaving the identically named outer variable unchanged).

Usually, the local scope references the local names of the (textually) current function. Outside functions, the local scope references the same namespace as the global scope: the module’s namespace. Class definitions place yet another namespace in the local scope.

It is important to realize that scopes are determined textually: the global scope of a function defined in a module is that module’s namespace, no matter from where or by what alias the function is called. On the other hand, the actual search for names is done dynamically, at run time — however, the language definition is evolving towards static name resolution, at “compile” time, so don’t rely on dynamic name resolution! (In fact, local variables are already determined statically.)

A special quirk of Python is that – if no global or nonlocal statement is in effect – assignments to names always go into the innermost scope. Assignments do not copy data — they just bind names to objects. The same is true for deletions: the statement del x removes the binding of x from the namespace referenced by the local scope. In fact, all operations that introduce new names use the local scope: in particular, import statements and function definitions bind the module or function name in the local scope.

The global statement can be used to indicate that particular variables live in the global scope and should be rebound there; the nonlocal statement indicates that particular variables live in an enclosing scope and should be rebound there.

9.2.1. Scopes and Namespaces Example¶

This is an example demonstrating how to reference the different scopes and namespaces, and how global and nonlocal affect variable binding:

The output of the example code is:

Note how the local assignment (which is default) didn’t change scope_test's binding of spam. The nonlocal assignment changed scope_test's binding of spam, and the global assignment changed the module-level binding.

You can also see that there was no previous binding for spam before the global assignment.

9.3. A First Look at Classes¶

Classes introduce a little bit of new syntax, three new object types, and some new semantics.

9.3.1. Class Definition Syntax¶

The simplest form of class definition looks like this:

Class definitions, like function definitions ( def statements) must be executed before they have any effect. (You could conceivably place a class definition in a branch of an if statement, or inside a function.)

In practice, the statements inside a class definition will usually be function definitions, but other statements are allowed, and sometimes useful — we’ll come back to this later. The function definitions inside a class normally have a peculiar form of argument list, dictated by the calling conventions for methods — again, this is explained later.

When a class definition is entered, a new namespace is created, and used as the local scope — thus, all assignments to local variables go into this new namespace. In particular, function definitions bind the name of the new function here.

When a class definition is left normally (via the end), a class object is created. This is basically a wrapper around the contents of the namespace created by the class definition; we’ll learn more about class objects in the next section. The original local scope (the one in effect just before the class definition was entered) is reinstated, and the class object is bound here to the class name given in the class definition header ( ClassName in the example).

9.3.2. Class Objects¶

Class objects support two kinds of operations: attribute references and instantiation.

Attribute references use the standard syntax used for all attribute references in Python: obj.name . Valid attribute names are all the names that were in the class’s namespace when the class object was created. So, if the class definition looked like this:

then MyClass.i and MyClass.f are valid attribute references, returning an integer and a function object, respectively. Class attributes can also be assigned to, so you can change the value of MyClass.i by assignment. __doc__ is also a valid attribute, returning the docstring belonging to the class: "A simple example class" .

Class instantiation uses function notation. Just pretend that the class object is a parameterless function that returns a new instance of the class. For example (assuming the above class):

creates a new instance of the class and assigns this object to the local variable x .

The instantiation operation (“calling” a class object) creates an empty object. Many classes like to create objects with instances customized to a specific initial state. Therefore a class may define a special method named __init__() , like this:

When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly-created class instance. So in this example, a new, initialized instance can be obtained by:

Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__() . For example,

9.3.3. Instance Objects¶

Now what can we do with instance objects? The only operations understood by instance objects are attribute references. There are two kinds of valid attribute names: data attributes and methods.

data attributes correspond to “instance variables” in Smalltalk, and to “data members” in C++. Data attributes need not be declared; like local variables, they spring into existence when they are first assigned to. For example, if x is the instance of MyClass created above, the following piece of code will print the value 16 , without leaving a trace:

The other kind of instance attribute reference is a method. A method is a function that “belongs to” an object. (In Python, the term method is not unique to class instances: other object types can have methods as well. For example, list objects have methods called append, insert, remove, sort, and so on. However, in the following discussion, we’ll use the term method exclusively to mean methods of class instance objects, unless explicitly stated otherwise.)

Valid method names of an instance object depend on its class. By definition, all attributes of a class that are function objects define corresponding methods of its instances. So in our example, x.f is a valid method reference, since MyClass.f is a function, but x.i is not, since MyClass.i is not. But x.f is not the same thing as MyClass.f — it is a method object, not a function object.

9.3.4. Method Objects¶

Usually, a method is called right after it is bound:

In the MyClass example, this will return the string 'hello world' . However, it is not necessary to call a method right away: x.f is a method object, and can be stored away and called at a later time. For example:

will continue to print hello world until the end of time.

What exactly happens when a method is called? You may have noticed that x.f() was called without an argument above, even though the function definition for f() specified an argument. What happened to the argument? Surely Python raises an exception when a function that requires an argument is called without any — even if the argument isn’t actually used…

Actually, you may have guessed the answer: the special thing about methods is that the instance object is passed as the first argument of the function. In our example, the call x.f() is exactly equivalent to MyClass.f(x) . In general, calling a method with a list of n arguments is equivalent to calling the corresponding function with an argument list that is created by inserting the method’s instance object before the first argument.

If you still don’t understand how methods work, a look at the implementation can perhaps clarify matters. When a non-data attribute of an instance is referenced, the instance’s class is searched. If the name denotes a valid class attribute that is a function object, a method object is created by packing (pointers to) the instance object and the function object just found together in an abstract object: this is the method object. When the method object is called with an argument list, a new argument list is constructed from the instance object and the argument list, and the function object is called with this new argument list.

9.3.5. Class and Instance Variables¶

Generally speaking, instance variables are for data unique to each instance and class variables are for attributes and methods shared by all instances of the class:

As discussed in A Word About Names and Objects , shared data can have possibly surprising effects with involving mutable objects such as lists and dictionaries. For example, the tricks list in the following code should not be used as a class variable because just a single list would be shared by all Dog instances:

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