我在这里做错了什么？ 我是python的新手

Question

Complete the body of the function sign (with signature below) in agreement with the details specified above.

 def sign(t): "Returns +1 for t>=0, -1 otherwise"

solution:

def sign(t):

sign = lambda t: -1 if t < 0 else 1 if t >=0

Above shows syntax error. What am I doing wrong here? it kind of shows name 'sign' not defined

Answer 1

Seems like all you need a is small function. Is this what you're looking for?

def sign(t):
    if t < 0:
        return -1
    else:
        return 1

Also, one thing is wrong with your lambda. If you expand it as if it was a normal function it would look like this:

def sign(t):
    if t < 0:
        return -1
    else:
       return 1 if t >= 0

So, there are only two cases. Either t is less than 0, or it is greater than or equal to zero. So, the following code will work:

sign = lambda t: -1 if t < 0 else 1
print(sign(2)) # will return 1

Answer 2

By itself,

def sign(t):
    "Returns +1 for t>=0, -1 otherwise"

is a syntactically complete function definition. It doesn't do anything, (except return None ) but the docstring is sufficient to make the body non-empty, as if you had defined it as

def sign(t):
    pass

instead.

What you need to do is add an actual body to the function so that it behaves the way the docstring describes.

def sign(t):
    "Returns +1 for t>=0, -1 otherwise"
    if t >= 0:
        return 1
    else:
        return -1

or

def sign(t):
    "Returns +1 for t>=0, -1 otherwise"
    return 1 if t >= 0 else -1