Using Yield and return a list of error

Question

I have a piece of code (an xls parser) that does some validation on the fields and returns with yield a generator that contains every row of the xls.

Now, i have to collect validation errors in a list, and use them when the generator is exhausted.

This is a piece of code that represent the parser and a poor designed solution.

error_list = []

def gen(limit): #xls parser
    for x in range(limit):
        if x%2: #fake error contition
            error_list.append(x)
        else:
            yield(x*x) #return

is there a more pythonic way to do this? i'm not a big fan of global variables.

i'd love to keep the code as it is as much as possible but if there's no other way i'll convert the function to a classic

def gen(limit): #xls parser
    error_list = []
    results = []
    for x in range(limit):
        if x%2: #fake error contition
            error_list.append(x)
        else:
            results.append(x*x)
    return results, error_list

Answer 1

A generator function cannot return out-of-band data like this.

I'd use a class instead, as an instance gives you something to stick such extra state:

class XLSParser(object):
    def __init__(self, limit):
        self.error_list = []
        self.limit = limit

    def __iter__(self):
        for x in range(self.limit):
            if x%2: #fake error condition
                self.error_list.append(x)
            else:
                yield(x*x) #return

and iterate over that object:

parser = XLSParser(limit)
for result in parser:
    # do something

errors = parser.error_list

Answer 2

You could use an exception:

class XlsErrorList(Exception): pass

def gen(limit):
    """xls parser"""
    error_list = []
    for x in range(limit):
        if x%2:  # fake error contition
            error_list.append(x)
        else:
            yield x*x

    if error_list:
        raise XlsErrorList(error_list)

try:
    for x in gen(10):
        print x

except XlsErrorList as e:
    print e.args