singledispatch_and_types.py

# errors.py
class NotPDFLinkError(Exception):

    def __str__(self):
        return "The Link Provided does not end with type `.pdf`"


class NotAnURLError(Exception):

    def __str__(self):
        return "The link provided is not a valid URL"


# pdf.py

@dataclass(frozen=True)
class ValidURL:
    string: str


@dataclass()
class ValidPDFURL:
    url: ValidURL


def get_valid_pdf_url(url) -> [ValidPDFURL, Exception]:
    return get_pdf_link(get_valid_url(url))


@singledispatch
def get_pdf_link(url) -> NotAnURLError:
    raise NotAnURLError()

@get_pdf_link.register
def _(url: ValidURL) -> [ValidPDFURL, NotPDFLinkError]:
    if not url.string.endswith('.pdf'):
        raise NotPDFLinkError()
    return ValidPDFURL(url)


@singledispatch
def get_valid_url(url) -> Exception:
    raise Exception('Not A String')


@get_valid_url.register
def _(url: str) -> [ValidURL, NotAnURLError]:
    result = urlparse(url)
    if not all([result.scheme, result.netloc, result.path]):
        raise NotAnURLError()
    return ValidURL(url)


@dataclass(frozen=True)
class SessionPDF:
    url: ValidPDFURL
    session: int
    session_date: datetime

    @property
    def filename(self):
        return f'session_{session}_{session_date.strftime("%d_%m_%Y")}.pdf'

    @property
    def link(self):
        return self.url.url.string


def download_pdf(session_pdf: SessionPDF, output_dir: str) -> str:
    if not os.path.isdir(output_dir): raise Exception('Output Directory is not a directory')

    data = requests.get(session_pdf.link).content
    output_path = os.path.join(output_dir, session_pdf.filename)
    with open(output_path, 'wb') as datafile:
        datafile.write(data)
    return output_path