Python - What does the “yield” keyword do?

素颜马尾好姑娘i 2022-05-29 06:43 125阅读 0赞

To understand what `yield` does, you must understand what generators are. And before generators come iterables.

# Iterables #

When you create a list, you can read its items one by one. Reading its items one by one is called iteration:

>>> mylist = [1, 2, 3]
    >>> for i in mylist:
    ...    print(i)
    1
    2
    3

`mylist` is an iterable. When you use a list comprehension, you create a list, and so an iterable:

>>> mylist = [x*x for x in range(3)]
    >>> for i in mylist:
    ...    print(i)
    0
    1
    4

Everything you can use "`for... in...`" on is an iterable; `lists`, `strings`, files...

These iterables are handy because you can read them as much as you wish, but you store all the values in memory and this is not always what you want when you have a lot of values.

# Generators #

Generators are iterators, a kind of iterable you can only iterate over once. Generators do not store all the values in memory, they generate the values on the fly:

>>> mygenerator = (x*x for x in range(3))
    >>> for i in mygenerator:
    ...    print(i)
    0
    1
    4

It is just the same except you used `()` instead of `[]`. BUT, you cannot perform `for i in mygenerator` a second time since generators can only be used once: they calculate 0, then forget about it and calculate 1, and end calculating 4, one by one.

# Yield #

`yield` is a keyword that is used like `return`, except the function will return a generator.

>>> def createGenerator():
    ...    mylist = range(3)
    ...    for i in mylist:
    ...        yield i*i
    ...
    >>> mygenerator = createGenerator() # create a generator
    >>> print(mygenerator) # mygenerator is an object!
    <generator object createGenerator at 0xb7555c34>
    >>> for i in mygenerator:
    ...     print(i)
    0
    1
    4

Here it's a useless example, but it's handy when you know your function will return a huge set of values that you will only need to read once.

To master `yield`, you must understand that when you call the function, the code you have written in the function body does not run. The function only returns the generator object, this is a bit tricky :-)

Then, your code will be run each time the `for` uses the generator.

Now the hard part:

The first time the `for` calls the generator object created from your function, it will run the code in your function from the beginning until it hits `yield`, then it'll return the first value of the loop. Then, each other call will run the loop you have written in the function one more time, and return the next value, until there is no value to return.

The generator is considered empty once the function runs but does not hit `yield` anymore. It can be because the loop had come to an end, or because you do not satisfy an `"if/else"` anymore.