在Python中强制/转换为正确类型的最佳位置 [英] Best place to coerce/convert to the right type in Python

问题描述

我对Python还是很陌生，我正在尝试适应它的动态类型.有时候，我有一个函数或一个类，它期望某种类型的参数，但是可以得到另一种可以强制执行的类型的值.例如，它可能期望为float，但接收的是int或小数.或者它可能需要一个字符串，但是会收到一个定义__str__特殊方法的对象.

I'm still fairly new to Python and I'm trying to get used to its dynamic typing. Sometimes I have a function or a class that expects a parameter of a certain type, but could get a value of another type that's coercible to it. For example, it might expect a float but instead receive an int or a decimal. Or it might expect a string, but instead receive an object that defines the __str__ special method.

将参数强制为正确类型(及其原因)的最佳实践是什么?我是在函数/类中还是在调用方中进行的?如果在调用方中，是否还要在函数中进行检查?例如

What is the best practice for coercing the argument to the right type (and the reason for it)? Do I do it in the function/class or in the caller? If in the caller, do I also check for it in the function? Eg.

替代1:

def myfunc(takes_float):
    myval = float(takes_float)

myfunc(5)

替代2:

def myfunc(takes_float):
    myval = takes_float

myfunc(float(5))

替代3:

def myfunc(takes_float):
    assert isinstance(takes_float, float)
    myval = takes_float

myfunc(float(5))

我已经阅读了此答案和

I've already read this answer and this one and they say that checking types in Python is "bad", but I don't want to waste time tracking down very simple bugs which would be instantly picked up by the compiler in a statically typed language.

推荐答案

在您不可或缺的情况下，您会强迫"(也许-可能是个小问题).例如，假设您有一个函数，该函数需要一个浮点数并返回其正弦和余弦的和:

You "coerce" (perhaps -- it could be a noop) when it's indispensable for you to do so, and no earlier. For example, say you have a function that takes a float and returns the sum of its sine and cosine:

import math
def spc(x):
  math.sin(x) + math.cos(x)

您应该在哪里强制" x浮动?答:根本没有地方-罪过和cos为您完成这项工作，例如:

Where should you "coerce" x to float? Answer: nowhere at all -- sin and cos do that job for you, e.g.:

>>> spc(decimal.Decimal('1.9'))
0.62301052082391117

那么什么时候强制(尽可能晚)强制进行 呢?例如，如果要在参数上调用字符串方法，则必须确保它是字符串-尝试调用例如在非字符串上的.lower将不起作用，如果arg例如，len可能会起作用，但与您期望的有所不同.一个列表(给您列表中的项数，而不是它表示为字符串的字符数)，等等.

So when it is indispensable to coerce (as late as possible)? For example, if you want to call string methods on an argument, you do have to make sure it's a string -- trying to call e.g. .lower on a non-string won't work, len might work but do something different than you expect if the arg is e.g. a list (give you the number of items in the list, not the number of characters its representation as a string will take up), and so forth.

对于捕获错误，请考虑单元测试-半体面的单元测试将捕获静态键入会出现的所有错误，然后再发现一些错误.但是，这是一个不同的主题.

As for catching errors -- think unit tests -- semidecent unit tests will catch all errors static typing would, and then some. But, that's a different subject.

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