Error while downloading NIST dataset

[s-rathi]

Describe the bug
The NISTWeightsComp and NISTIonizationEnergies functions are unstable and fail at some executions due to SSLCertVerificationError. The functions start working again on repeated attempts. The error occurs when the program tries to fetch the data from https://physics.nist.gov/.

To Reproduce

1. Configure the project and launch the python shell.
2. Run these commands:

from carsus.io.nist import NISTIonizationEnergies
ionization_energies = NISTIonizationEnergies('H-Zn')

The NISTIonizationEnergies() function can fail randomly.

System

• OS:

  • GNU/Linux
  • macOS

• Environment (conda list): carsus

Additional context
Error logs:

>>> ionization_energies = NISTIonizationEnergies('H-Zn')
[  carsus.io.nist.ionization][   INFO] - Downloading ionization energies from the NIST Atomic Spectra Database. (ionization.py:64)
Traceback (most recent call last):
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/urllib3/connectionpool.py", line 710, in urlopen
    chunked=chunked,
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/urllib3/connectionpool.py", line 386, in _make_request
    self._validate_conn(conn)
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/urllib3/connectionpool.py", line 1042, in _validate_conn
    conn.connect()
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/urllib3/connection.py", line 424, in connect
    tls_in_tls=tls_in_tls,
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/urllib3/util/ssl_.py", line 450, in ssl_wrap_socket
    sock, context, tls_in_tls, server_hostname=server_hostname
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/urllib3/util/ssl_.py", line 493, in _ssl_wrap_socket_impl
    return ssl_context.wrap_socket(sock, server_hostname=server_hostname)
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/ssl.py", line 423, in wrap_socket
    session=session
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/ssl.py", line 870, in _create
    self.do_handshake()
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/ssl.py", line 1139, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1091)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/requests/adapters.py", line 499, in send
    timeout=timeout,
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/urllib3/connectionpool.py", line 788, in urlopen
    method, url, error=e, _pool=self, _stacktrace=sys.exc_info()[2]
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/urllib3/util/retry.py", line 592, in increment
    raise MaxRetryError(_pool, url, error or ResponseError(cause))
urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='physics.nist.gov', port=443): Max retries exceeded with url: /PhysRefData/ASD/Html/verhist.shtml (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1091)')))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/shikha/Dev/Proj/carsus/carsus/io/nist/ionization.py", line 319, in __init__
    self._get_version()
  File "/Users/shikha/Dev/Proj/carsus/carsus/io/nist/ionization.py", line 355, in _get_version
    html = requests.get(IONIZATION_ENERGIES_VERSION_URL)
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/requests/api.py", line 73, in get
    return request("get", url, params=params, **kwargs)
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/requests/api.py", line 59, in request
    return session.request(method=method, url=url, **kwargs)
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/requests/sessions.py", line 587, in request
    resp = self.send(prep, **send_kwargs)
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/requests/sessions.py", line 701, in send
    r = adapter.send(request, **kwargs)
  File "/Users/shikha/miniconda3/envs/carsus/lib/python3.7/site-packages/requests/adapters.py", line 563, in send
    raise SSLError(e, request=request)
requests.exceptions.SSLError: HTTPSConnectionPool(host='physics.nist.gov', port=443): Max retries exceeded with url: /PhysRefData/ASD/Html/verhist.shtml (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1091)')))