查找有关python“本地”类型的文档，例如set

Question

I'm trying to learn Python, and, while I managed to stumble on the answer to my current problem, I'd like to know how I can better find answers in the future.

My goal was to take a list of strings as input, and return a string whose characters were the union of the characters in the strings, eg

unionStrings( ("ab", "bc"))

would return "abc".

I implemented it like this:

def unionStrings( strings ):
     # Input: A list of strings
     # Output: A string that is the (set) union of input strings
    all = set()
    for s in strings:
         all = all.union(set(s))

    return "".join(sorted(list(all)))

I felt the for loop was unnecessary, and searched for more neater, more pythonic(?), improvements .

First question : I stumbled on using the class method set.union() , instead of set1.union(set2) . Should I have been able to find this in the standard Python docs? I've not been able to find it there.

So I tried using set.union() like this:

>>> set.union( [set(x) for x in ("ab","bc")] )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: descriptor 'union' requires a 'set' object but received a 'list'

Again, I stumbled around and finally found that I should be calling it like this:

>>> set.union( *[set(x) for x in ("ab","bc")] )
set(['a', 'c', 'b'])

Second question : I think this means that set.union is (effectively) declared as

set.union( *sets)

and not

set.union( setsList ) 

Is that correct? (I'm still learning how to use splat '*'.)

Third question : Where could I find documentation on the signature of set.union() ? I didn't see it in the set/freezeset doc's, and I couldn't get the inspect module to give me anything. I'm not even sure set is a module, it seems to be a type. Is it defined in a module, or what?

Thanks for reading my complicated question. It's more "How do I navigate Python documentation?" than "How do I do this in Python code?".

---

Responding to jonrsharpe's comment:

Ohhhhh! I'm so used to C++ where you define separate static and instance methods. Now that you explain it I can really see what's happening.

The only thing I might do different is write it as

t = set.union( *[set(x) for x strings] )
return "".join(sorted(t))

because it bugs me to treat strings[0] differently from the strings in strings[1:] when, functionally, they don't play different roles. If I have to call set() on one of them, I'd rather call it on all of them, since union() is going to do it anyways. But that's just style, right?

Answer 1

There are several questions here. Firstly, you should know that:

Class.method(instance, arg)

is equivalent to:

instance.method(arg)

for instance methods . You can call the method on the class and explicitly provide the instance, or just call it on the instance.

For historical reasons, many of the standard library and built-in types don't follow the UppercaseWords convention for class names, but they are classes . Therefore

set.union(aset, anotherset)

is the same as

aset.union(anotherset)

set methods specifically can be tricky, because of the way they're often used. set.method(arg1, arg2, ...) requires arg1 to already be a set , the instance for the method, but all the other arguments will be converted (from 2.6 on).

This isn't directly covered in the set docs, because it's true for everything; Python is pretty consistent.

In terms of needing to "splat", note that the docs say:

union(other, ...)

rather than

union(others)

ie each iterable is a separate argument, hence you need to unpack your list of iterables.

---

Your function could therefore be:

def union_strings(strings):
    if not strings:
        return ""
    return "".join(sorted(set(strings[0]).union(*strings[1:])))

or, avoiding the special-casing of strings[0] :

def union_strings(strings): 
    if not strings:
        return ""
    return "".join(sorted(set.union(*map(set, strings))))