bug with invalid redir

[wumpus]

This host lacks a Location: in this redir -- a not unusual thing on the Internets:

$ curl -D /dev/tty https://www.enterprisecarshare.com/robots.txt
HTTP/1.1 302 Moved Temporarily
Content-Type: text/html;charset=UTF-8
Dispatcher: dispatcher 1d
Server: Apache
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
Content-Length: 0
Cache-Control: max-age=168018
Expires: Tue, 15 Nov 2016 02:30:09 GMT
Date: Sun, 13 Nov 2016 03:49:51 GMT
Connection: keep-alive

I updated to 1.1.0, and if there's allow_redirects=True the code now calls yarl.URL(None) which throws

 TypeError: Constructor parameter should be str

which is not so super-useful to the programmer! May I suggest instead looking to see if we're about to pass None into yarl.URL and in this case raise ValueError(url + ' is a redirect but lacks a Location: or URI: header')

$ python bug.py
Traceback (most recent call last):
  File "bug.py", line 11, in <module>
    loop.run_until_complete(main(url))
  File "/home/lindahl/.pyenv/versions/3.5.2/lib/python3.5/asyncio/base_events.py", line 387, in run_until_complete
    return future.result()
  File "/home/lindahl/.pyenv/versions/3.5.2/lib/python3.5/asyncio/futures.py", line 274, in result
    raise self._exception
  File "/home/lindahl/.pyenv/versions/3.5.2/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "bug.py", line 6, in main
    async with session.get(url, allow_redirects=True) as response:
  File "/home/lindahl/.pyenv/versions/3.5.2/lib/python3.5/site-packages/aiohttp/client.py", line 558, in __aenter__
    self._resp = yield from self._coro
  File "/home/lindahl/.pyenv/versions/3.5.2/lib/python3.5/site-packages/aiohttp/client.py", line 245, in _request
    resp.headers.get(hdrs.URI))
  File "/home/lindahl/.pyenv/versions/3.5.2/lib/python3.5/site-packages/yarl/__init__.py", line 150, in __init__
    raise TypeError("Constructor parameter should be str")
TypeError: Constructor parameter should be str

$ more bug.py
import asyncio
import aiohttp

async def main(url):
    async with aiohttp.ClientSession() as session:
        async with session.get(url, allow_redirects=True) as response:
            await response.read()

url = 'https://www.enterprisecarshare.com/robots.txt'
loop = asyncio.get_event_loop()
loop.run_until_complete(main(url))

[asvetlov]

Makes sense.

Would you make a Pull Request?

[asvetlov]

Fixed by f137b18

Now it raises RuntimeError with text like 'GET http://127.0.0.1:56346/redirect returns a redirect [301] status but response lacks a Location or URI HTTP header'.
Exception type is RuntimeError because ValueError is about wrong value passed to function but for this case real error source is a server answer.

@wumpus is error text looks good and informative enough?

[wumpus]

Thank you, that looks perfect.

(Thanks for the test example, I haven't gotten through the learning curve of your test suite yet)

[asvetlov]

The test suite is based on pytest tool with adding several pytest fixtures and hooks.
It's described by http://aiohttp.readthedocs.io/en/stable/testing.html but I pretty sure the chapter is not perfect. It needs a love for polishing.

Please make a Pull Request (or just an issue) with proposal for solving dark documentation's corners.

I really need feedbacks for documentation.
Maybe somebody will become an awesome docs writer for the project but now it's just a dream.

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