read_sas catches own error

[samgd]

Code Sample, a copy-pastable example if possible

import pandas as pd
pd.read_sas('/tmp/foo')

Output (/tmp/foo does not have to exist):

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-10-de3f5c15fb71> in <module>
      1 import pandas as pd
----> 2 pd.read_sas('/tmp/foo')

~/.virtualenvs/pandas/lib/python3.7/site-packages/pandas/io/sas/sasreader.py in read_sas(filepath_or_buffer, format, index, encoding, chunksize, iterator)
     50             pass
     51 
---> 52     if format.lower() == 'xport':
     53         from pandas.io.sas.sas_xport import XportReader
     54         reader = XportReader(filepath_or_buffer, index=index,

AttributeError: 'NoneType' object has no attribute 'lower'

Problem description

When format=None the function attempts to infer the file's format by naively looking at the extension. If it is neither '.xpt' nor '.sas7bdat' then an error is raised. However, this logic is wrapped in a try/except block that catches the raised error and silently discards it:

pandas/pandas/io/sas/sasreader.py

Lines 35 to 53 in 179822a

	if format is None:
	buffer_error_msg = ("If this is a buffer object rather "
	"than a string name, you must specify "
	"a format string")
	filepath_or_buffer = _stringify_path(filepath_or_buffer)
	if not isinstance(filepath_or_buffer, compat.string_types):
	raise ValueError(buffer_error_msg)
	try:
	fname = filepath_or_buffer.lower()
	if fname.endswith(".xpt"):
	format = "xport"
	elif fname.endswith(".sas7bdat"):
	format = "sas7bdat"
	else:
	raise ValueError("unable to infer format of SAS file")
	except ValueError:
	pass
	if format.lower() == 'xport':

The function then attempts to call format.lower() but format is still None so an AttributeError is raised.

Expected Output

Expected behaviour: error raised when unable to infer format. Suggest documenting that inference is done by checking the file's extension.

ValueError: unable to infer format of SAS file

Output of pd.show_versions()

[paste the output of pd.show_versions() here below this line]
INSTALLED VERSIONS

commit: None
python: 3.7.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.19.12-arch1-1-ARCH
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.23.4
pytest: None
pip: 18.1
setuptools: 40.6.3
Cython: None
numpy: 1.15.4
scipy: None
pyarrow: None
xarray: None
IPython: 7.2.0
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[benjaminr]

I see what you mean. Could quickly be resolved by swapping pass to raise and Updating the docstring. I'll add a PR for these changes.

[samgd]

Is the try/except clause even required?filepath_or_buffer is a string given the above guard statement so both lower() and endswith should be error free?

[samgd]

If the try statement is there to catch something in the logic I've missed then changing it to:

try:
     ...
except ValueError:
    raise

is odd as this is essentially a noop?

[benjaminr]

@samgd yeah you're right. Will update to reflect that.