73 lines
2.9 KiB
Markdown
73 lines
2.9 KiB
Markdown
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# Python 构造函数
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> 原文:[https://python.land/objects-and-classes/python-constructors](https://python.land/objects-and-classes/python-constructors)
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我们现在来看一个特殊的 [Python 函数](https://python.land/introduction-to-python/functions)，它是大多数类的一部分:Python 构造函数。构造函数是在创建对象时自动调用的函数。构造函数也可以有选择地接受参数，就像常规函数一样。
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目录
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* [默认构造函数](#The_default_constructor "The default constructor")
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* [创建自己的 Python 构造函数](#Creating_your_own_Python_constructor "Creating your own Python constructor")
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## 默认构造函数
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当从一个类创建一个对象时，看起来我们在调用一个函数:
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```py
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car = Car()
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```
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嗯…这不仅仅看起来像是我们在调用一个函数，我们实际上是在调用一个函数！我们不必定义这个方法，它被称为构造函数。它构造并初始化对象。每个类默认都有一个，叫做`__init__`，即使我们自己没有定义。这与[继承](https://python.land/objects-and-classes/python-inheritance)有关，你很快就会了解到。
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你用过`str()`函数把数字转换成字符串吗？或者是将字符串转换成数字的`int()`函数？
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```py
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>>> 'a' + str(1)
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'a1'
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>>> int('2') + 2
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4
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```
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你在这里所做的，是通过调用类`str`和`int`的构造函数来创建类型`str`和`int`的新对象。
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## 创建自己的 Python 构造函数
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我们可以覆盖`__init__`方法，通过接受参数赋予它额外的能力。让我们使用自定义构造函数重新定义`Car`类:
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```py
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class Car:
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def __init__(self, started = False, speed = 0):
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self.started = started
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self.speed = speed
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def start(self):
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self.started = True
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print("Car started, let's ride!")
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def increase_speed(self, delta):
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if self.started:
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self.speed = self.speed + delta
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print("Vrooooom!")
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else:
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print("You need to start the car first")
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def stop(self):
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self.speed = 0
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```
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我们的定制 Python 构造函数有带默认值的命名参数[，所以我们可以用多种方式创建类`Car`的实例:](https://python.land/introduction-to-python/functions#Default_values_and_named_parameters)
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```py
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>>> c1 = Car()
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>>> c2 = Car(True)
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>>> c3 = Car(True, 50)
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>>> c4 = Car(started=True, speed=40)
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```
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你可能已经注意到了一个缺陷:我们现在可以创造一辆新车，它不启动，但无论如何都要设定它的速度。现在，我们就到此为止吧。
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