io.ascii guessing tries formats it doesn't need to

I made a file that is not a valid ASCII table for testing and tried to read it in using:

>>> from astropy.io.ascii import read
>>> read('test', delimiter=',')

I noticed that some of the formats that were tried are ones for which delimiter=',' shouldn’t be a valid option:

InconsistentTableError: 
ERROR: Unable to guess table format with the guesses listed below:
Reader:Ecsv delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:FixedWidthTwoLine delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:RST delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:FastBasic delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:Basic delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:FastRdb delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:Rdb delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:FastTab delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:Tab delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:Cds delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:Daophot delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:SExtractor delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:Ipac delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:Latex delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:AASTex delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] strict_names: True
Reader:FastCommentedHeader delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: '"' strict_names: True
Reader:FastCommentedHeader delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: "'" strict_names: True
Reader:CommentedHeader delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: '"' strict_names: True
Reader:CommentedHeader delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: "'" strict_names: True
Reader:FastBasic delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: '"' strict_names: True
Reader:FastBasic delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: "'" strict_names: True
Reader:Basic delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: '"' strict_names: True
Reader:Basic delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: "'" strict_names: True
Reader:FastNoHeader delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: '"' strict_names: True
Reader:FastNoHeader delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: "'" strict_names: True
Reader:NoHeader delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: '"' strict_names: True
Reader:NoHeader delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')] quotechar: "'" strict_names: True
Reader:Basic delimiter: ',' fast_reader: {'enable': True} fill_values: [('', '0')]

************************************************************************
** ERROR: Unable to guess table format with the guesses listed above. **
**                                                                    **
** To figure out why the table did not read, use guess=False and      **
** fast_reader=False, along with any appropriate arguments to read(). **
** In particular specify the format and any known attributes like the **
** delimiter.                                                         **
************************************************************************

Specifically, Ipac and Tab shouldn’t be tried since delimiter=',' shouldn’t be a valid option for those? I thought maybe it was still skipping these internally but the read trace shows that they are actually executed:

 {'kwargs': {'Reader': astropy.io.ascii.ipac.Ipac,
   'delimiter': ',',
   'fast_reader': {'enable': True},
   'fill_values': [('', '0')],
   'strict_names': True},
  'status': 'ValueError: At least one header line beginning and ending with delimiter required',
  'dt': '3.011 ms'},

There might be room for optimization here by checking whether the reader actually supports the specified options?

[hamogu]

We really don’t know either, since we don’t count which function is called how often by which user. You’ll also notice that he most specific (ESCV, LATEX) readers are on the top. Those should fail fast, while the generic readers that read almost anything are at the bottom (That’s because a table that is ECSV will almost certainly be ECSV, while a table that the CSV can parse could also be e.g. a Daophot table.)

All in all, this is a complex issue where a lot of obscure corner cases will come up. I’m not saying @bhavyakh can’t do it - but it’s probably much more involved than it sounds at first!

[bhavyakh]

Yes! I tried looking at some tests and the reader functions, I got a vague idea of how many issues can come up. I totally understand your point.