Python yield (migrating from Ruby): How can I write a function without arguments and only with yield to do prints?

Question

I've been converting Ruby code to Python code and now I'm stuck with this function that contains yield:

def three_print():
    yield
    yield
    yield

I would like to call the function and tell it to print "Hello" three times because of the three yield statements. As the function does not take any arguments I get an error. Can you tell me the easiest way to get it working? Thank you.

Answer 1

yield in Ruby and yield in Python are two very different things.

In Ruby yield runs a block passed as a parameter to the function.

Ruby:

def three
  yield
  yield
  yield
end

three { puts 'hello '} # runs block (prints "hello") three times

In Python yield throws a value from a generator (which is a function that uses yield) and stops execution of the function. So it's something completely different, more likely you want to pass a function as a parameter to the function in Python.

Python:

def three(func):
  func()
  func()
  func()

three(lambda: print('hello')) # runs function (prints "hello") three times

Python Generators

The code below (code you've provided) is a generator which returns None three times:

def three():
   yield
   yield
   yield

g = three() #=> <generator object three at 0x7fa3e31cb0a0>
next(g) #=> None
next(g) #=> None
next(g) #=> None
next(g) #=> StopIteration

The only way that I can imagine how it could be used for printing "Hello" three times -- using it as an iterator:

for _ in three():
    print('Hello')

Ruby Analogy

You can do a similar thing in Ruby using Enumerator.new:

def three
  Enumerator.new do |e|
    e.yield # or e << nil
    e.yield # or e << nil
    e.yield # or e << nil
  end
end

g = three
g.next #=> nil
g.next #=> nil
g.next #=> nil
g.next #=> StopIteration

three.each do
  puts 'Hello'
end