Question

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The Python yield keyword explained

Okay, I’ve probably phrased the question badly but this is the situation I have.

I have this line of code in Python 2.7 which I’m trying to understand:

yield (padding_zeros + number_string).encode("ascii")

In this line of code, padding_zeros is a string of a variable number of ‘0’s and number_string is a number in the form of a string which can be any number between 0 to, say 10000.

I’m pretty confident that the .encode("ascii") just converts the output of yield to ascii.

What I’m completely at sea about is what the yield (padding_zeros + number_string) does.

I know it initiates a generator but I’ve spent a lot of time searching online and reading up on the syntax but I still can’t work out what the generator actually does. It doesn’t help that this is my first time looking at python (my ultimate aim is to convert this code to C#).

So, basically, please could someone explain to me what this line of code does? Does it just add the two strings together or does it do something a bit more complicated?

For further context, this is the block that that line of code appears in:

for current_length in range(4, max_length + 1):
    for i in range(0, pow(10, current_length)):
        number_string = str(i)
        padding_zeros = "0" * (current_length - len(number_string))
        yield (padding_zeros + number_string).encode("ascii")

(max_length being exactly what it sounds like – a number indicating the maximum length of something)

Thanks in advance for any and all answers (even if they’re telling me not to be such a fricking noob) 🙂

EDIT: Thanks very much for the answers – even though I could only pick one as the best answe they were all very helpful. And thanks for the comments as well – as some of them pointed out, What does the "yield" keyword do in Python? is a very good general guide to yield, generators and iterations even if I didn’t find it an answer to my specific situation 🙂

Answer 1

OK, you know about generators, so the yield part needs no explanation. Fine.

So what does that line actually do? Not very much:

It concatenates padding_zeros and number_string and then encodes the result to ASCII. Which in Python 2.7 is a no-op because the string is ASCII to begin with (it only consists of ASCII digits, by definition).

In Python 3, it would be different; here the .encode() would have converted the string to a bytes object. But in Python 2, it doesn’t make any sense.