Python – What does yield do in python 2.7?

generatorpythonpython-2.7stringyield

Possible Duplicate:
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 🙂

Best Answer

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.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – the yield keyword used for in C#

The yield contextual keyword actually does quite a lot here.

The function returns an object that implements the IEnumerable<object> interface. If a calling function starts foreaching over this object, the function is called again until it "yields". This is syntactic sugar introduced in C# 2.0. In earlier versions you had to create your own IEnumerable and IEnumerator objects to do stuff like this.

The easiest way understand code like this is to type-in an example, set some breakpoints and see what happens. Try stepping through this example:

public void Consumer()
{
    foreach(int i in Integers())
    {
        Console.WriteLine(i.ToString());
    }
}

public IEnumerable<int> Integers()
{
    yield return 1;
    yield return 2;
    yield return 4;
    yield return 8;
    yield return 16;
    yield return 16777216;
}

When you step through the example, you'll find the first call to Integers() returns 1. The second call returns 2 and the line yield return 1 is not executed again.

Here is a real-life example:

public IEnumerable<T> Read<T>(string sql, Func<IDataReader, T> make, params object[] parms)
{
    using (var connection = CreateConnection())
    {
        using (var command = CreateCommand(CommandType.Text, sql, connection, parms))
        {
            command.CommandTimeout = dataBaseSettings.ReadCommandTimeout;
            using (var reader = command.ExecuteReader())
            {
                while (reader.Read())
                {
                    yield return make(reader);
                }
            }
        }
    }
}

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – the yield keyword used for in C#

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