在 Python 中断言变量类型的正确方法 [英] Proper way to assert type of variable in Python

问题描述

在使用函数时，我希望确保变量的类型符合预期.怎么做才对?

In using a function, I wish to ensure that the type of the variables are as expected. How to do it right?

下面是一个假函数的例子，它在继续其角色之前试图做到这一点:

Here is an example fake function trying to do just this before going on with its role:

def my_print(begin, text, end):
    """Print 'text' in UPPER between 'begin' and 'end' in lower

    """
    for i in (begin, text, end):
        assert isinstance(i, str), "Input variables should be strings"
    out = begin.lower() + text.upper() + end.lower()
    print out

def test():
    """Put your test cases here!

    """
    assert my_print("asdf", "fssfpoie", "fsodf")
    assert not my_print("fasdf", 33, "adfas")
    print "All tests passed"

test()

断言是正确的方法吗?我应该改用 try/except 吗?

Is assert the right approach? Should I use try/except instead?

此外，我的断言测试集似乎无法正常工作:S

Also, my assert set of tests does not seem to work properly :S

感谢 pythoneers

Thanks pythoneers

推荐答案

isinstance 如果你真的必须，内置是首选的方式，但更好的是记住 Python 的座右铭:请求宽恕比许可更容易"！-)(这实际上是 Grace Murray Hopper 的最爱座右铭;-).即:

The isinstance built-in is the preferred way if you really must, but even better is to remember Python's motto: "it's easier to ask forgiveness than permission"!-) (It was actually Grace Murray Hopper's favorite motto;-). I.e.:

def my_print(text, begin, end):
    "Print 'text' in UPPER between 'begin' and 'end' in lower"
    try:
      print begin.lower() + text.upper() + end.lower()
    except (AttributeError, TypeError):
      raise AssertionError('Input variables should be strings')

顺便说一句，这让函数可以在 Unicode 字符串上正常工作——无需任何额外的努力！-)

This, BTW, lets the function work just fine on Unicode strings -- without any extra effort!-)

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