Understanding the asterisk operator in python when it's before the function in a parenthesis-CodePudding

I know that the asterisk is used to unpack values like system args or when you unpack lists into variables.

But I have not seen this syntax here before in this example of asyncio.

I was reading this article here, https://realpython.com/async-io-python/#the-10000-foot-view-of-async-io , but I don't understand what the asterisk operator is doing in this context.

#!/usr/bin/env python3
# rand.py

import asyncio
import random

# ANSI colors
c = (
    "\033[0m",   # End of color
    "\033[36m",  # Cyan
    "\033[91m",  # Red
    "\033[35m",  # Magenta
)

async def makerandom(idx: int, threshold: int = 6) -> int:
    print(c[idx   1]   f"Initiated makerandom({idx}).")
    i = random.randint(0, 10)
    while i <= threshold:
        print(c[idx   1]   f"makerandom({idx}) == {i} too low; retrying.")
        await asyncio.sleep(idx   1)
        i = random.randint(0, 10)
    print(c[idx   1]   f"---> Finished: makerandom({idx}) == {i}"   c[0])
    return i

async def main():
    res = await asyncio.gather(*(makerandom(i, 10 - i - 1) for i in range(3)))
    return res

if __name__ == "__main__":
    random.seed(444)
    r1, r2, r3 = asyncio.run(main())
    print()
    print(f"r1: {r1}, r2: {r2}, r3: {r3}")

Under the async def main function right before the makerandom there is an asterisk. Could someone explain what it does in this context ? I am trying to understand how async / await works.

I looked at this answer, Python asterisk before function but it doesn't really explain it.

CodePudding user response：

The asterisk isn't before makerandom, it's before the generator expression

(makerandom(i, 10 - i - 1) for i in range(3))

asyncio.gather doesn't take an iterable as its first argument; it accepts a variable number of awaitables as positional arguments. In order to get from a generator expression to that, you need to unpack the generator.

In this particular instance, the asterisk unpacks

asyncio.gather(*(makerandom(i, 10 - i - 1) for i in range(3)))

into

asyncio.gather(makerandom(0, 9), makerandom(1, 8), makerandom(2, 7))

CodePudding user response：

An asterisk, means unpacking. It spreads the iterable into several arguments as in:

array = [1, 2, 3]
print(*array)

would be the same as

print(1, 2, 3)

Another example:

generator = (x for x in range(10) if my_function(x))
print(*generator)

One asterisk is being used for ordinary iterables, but when it comes to mappings, you can also use double-asterisk as ** in order to unpack the mapping into keyword arguments, as in:

dictionary = {"hello": 1, "world": 2}
my_function(**dictionary)

This would be equivalent to my_function(hello=1, world=2). You can also use the ordinary one-asterisk in order to only unpack the keys from the mapping:

print(*{"hello": 1, "world": 2})

This would result in hello world