Somewhat cryptic SyntaxError for seemingly ok attribute name

[xrotwang]

Some attribute nams which are ok for python are not ok for attrs:

>>> import attr
>>> @attr.s
... class A(object):
...     _1 = attr.ib()
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File ".../lib/python2.7/site-packages/attr/_make.py", line 302, in attributes
    return wrap(maybe_cls)
  File ".../lib/python2.7/site-packages/attr/_make.py", line 274, in wrap
    cls = _add_init(cls, frozen)
  File ".../lib/python2.7/site-packages/attr/_make.py", line 453, in _add_init
    bytecode = compile(script, unique_filename, "exec")
  File "<attrs generated init 59333f495b05c926d5d0a4a1c3ea144621198fa1>", line 1
    def __init__(self, 1):
                       ^
SyntaxError: invalid syntax

I know what's going wrong here, but the error is still somewhat cryptic. So I guess this is mostly a documentation issue. But it puzzled me for some time, because I started out with column names in a CSV file - one of them starting with a "2". So pycharm alerted me of the fact that python names cannot start with a number. A "_" prefix made pycharm happy, but upon import of the module, I ran into this issue.

The error I get for a similar case with namedtuple seems more transparent:

>>> from collections import namedtuple
>>> c = namedtuple('c', ['1'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/collections.py", line 337, in namedtuple
    'a number: %r' % name)
ValueError: Type names and field names cannot start with a number: '1'

[wbolster]

fwiw, on py3 this may help:

import keyword

def is_valid_attribute_name(s):
    return s.isidentifier() and not keyword.iskeyword(s)

[hynek]

fixed in #370