Default values for iterable unpacking

问题

I've often been frustrated by the lack of flexibility in Python's iterable unpacking.

Take the following example:

a, b = range(2)

Works fine. a contains 0 and b contains 1, just as expected. Now let's try this:

a, b = range(1)

Now, we get a ValueError:

ValueError: not enough values to unpack (expected 2, got 1)

Not ideal, when the desired result was 0 in a, and None in b.

There are a number of hacks to get around this. The most elegant I've seen is this:

a, *b = function_with_variable_number_of_return_values()
b = b[0] if b else None

Not pretty, and could be confusing to Python newcomers.

So what's the most Pythonic way to do this? Store the return value in a variable and use an if block? The *varname hack? Something else?

回答1:

As mentioned in the comments, the best way to do this is to simply have your function return a constant number of values and if your use case is actually more complicated (like argument parsing), use a library for it.

However, your question explicitly asked for a Pythonic way of handling functions that return a variable number of arguments and I believe it can be cleanly accomplished with decorators. They're not super common and most people tend to use them more than create them so here's a down-to-earth tutorial on creating decorators to learn more about them.

Below is a decorated function that does what you're looking for. The function returns an iterator with a variable number of arguments and it is padded up to a certain length to better accommodate iterator unpacking.

def variable_return(max_values, default=None):
    # This decorator is somewhat more complicated because the decorator
    # itself needs to take arguments.
    def decorator(f):
        def wrapper(*args, **kwargs):
            actual_values = f(*args, **kwargs)
            try:
                # This will fail if `actual_values` is a single value.
                # Such as a single integer or just `None`.
                actual_values = list(actual_values)
            except:
                actual_values = [actual_values]
            extra = [default] * (max_values - len(actual_values))
            actual_values.extend(extra)
            return actual_values
        return wrapper
    return decorator

@variable_return(max_values=3)
# This would be a function that actually does something.
# It should not return more values than `max_values`.
def ret_n(n):
    return list(range(n))

a, b, c = ret_n(1)
print(a, b, c)
a, b, c = ret_n(2)
print(a, b, c)
a, b, c = ret_n(3)
print(a, b, c)

Which outputs what you're looking for:

0 None None
0 1 None
0 1 2

The decorator basically takes the decorated function and returns its output along with enough extra values to fill in max_values. The caller can then assume that the function always returns exactly max_values number of arguments and can use fancy unpacking like normal.

回答2:

Here's an alternative version of the decorator solution by @supersam654, using iterators rather than lists for efficiency:

def variable_return(max_values, default=None):
    def decorator(f):
        def wrapper(*args, **kwargs):
            actual_values = f(*args, **kwargs)
            try:
                for count, value in enumerate(actual_values, 1):
                    yield value
            except TypeError:
                count = 1
                yield actual_values
            yield from [default] * (max_values - count)
        return wrapper
    return decorator

It's used in the same way:

@variable_return(3)
def ret_n(n):
    return tuple(range(n))

a, b, c = ret_n(2)

This could also be used with non-user-defined functions like so:

a, b, c = variable_return(3)(range)(2)

来源：https://stackoverflow.com/questions/44618351/default-values-for-iterable-unpacking

标签

python

python-3.x

iterable-unpacking