Question

(So I’m trying to learn python. I figured it would be good to read code by people better than me. I decided to read through the email module…)

The init function for the Feedparser class in the email.feedparser module is defined as:

def __init__(self, _factory=message.Message):
    """_factory is called with no arguments to create a new message obj"""
    self._factory = _factory
    self._input = BufferedSubFile()
    self._msgstack = []
    self._parse = self._parsegen().next
    self._cur = None
    self._last = None
    self._headersonly = False

The line I’m having trouble with is:

self._parse = self._parsegen().next

Which I think should mean ‘set the attribute self._parse to the value of the next attribute of the return value of the method self._parsegen()

As far as I can tell, self._parsgen() when called during __init__() will first call self._new_message() which will set/add values to self._cur, self._last, and self._msgstack. It will then assign an empty list object to the local variable headers then start iterating over the self._input object. I think the first value for line will be a NeedMoreData object. Since the NeedMoreData class just extends object it should have no attribute or method named next. So does next just refer back to the iterator (self._input)?

Is there any way to have a look at this in the interpreter so that I can step through each line of the script?

Answer 1

So does next just refer back to the iterator (self._input)?

next does refer to the generator. Since the _parsegen() method uses yield, it returns a generator object. Consider the following simple example (from IPython):

In [1]: def a():
   ...:     yield 1
   ...:     yield 2
   ...:     

In [2]: a()
Out[2]: <generator object a at 0x1a56550>

In [3]: a().next
Out[3]: <method-wrapper 'next' of generator object at 0x1a567d0>

In [4]: a().next()
Out[4]: 1

So, yes, you are mostly right. It will fall down to the iterator, and reference the method returning the next value from it.

Is there any way to have a look at this in the interpreter so that I can step through each line of the script?

You can use pdb for that.