Clean up how pvlib imports optional dependencies

pvlib/iotools/ecmwf_macc.py

@@ -4,24 +4,14 @@
 
 import threading
 import pandas as pd
+from pvlib.tools import _optional_import
+
+netCDF4 = _optional_import('netCDF4', (
+    'Reading ECMWF data requires netCDF4 to be installed.'))
+ecmwfapi = _optional_import('ecmwfapi', (
+    'To download data from ECMWF requires the API client.\nSee https:/'
+    '/confluence.ecmwf.int/display/WEBAPI/Access+ECMWF+Public+Datasets'))
 
-try:
-    import netCDF4
-except ImportError:
-    class netCDF4:
-        @staticmethod
-        def Dataset(*a, **kw):
-            raise ImportError(
-                'Reading ECMWF data requires netCDF4 to be installed.')
-
-try:
-    from ecmwfapi import ECMWFDataServer
-except ImportError:
-    def ECMWFDataServer(*a, **kw):
-        raise ImportError(
-            'To download data from ECMWF requires the API client.\nSee https:/'
-            '/confluence.ecmwf.int/display/WEBAPI/Access+ECMWF+Public+Datasets'
-        )
 
 #: map of ECMWF MACC parameter keynames and codes used in API
 PARAMS = {
@@ -164,7 +154,7 @@ def get_ecmwf_macc(filename, params, start, end, lookup_params=True,
     startdate = start.strftime('%Y-%m-%d')
     enddate = end.strftime('%Y-%m-%d')
     if not server:
-        server = ECMWFDataServer()
+        server = ecmwfapi.ECMWFDataServer()
     t = threading.Thread(target=target, daemon=True,
                          args=(server, startdate, enddate, params, filename))
     t.start()

pvlib/solarposition.py

@@ -24,7 +24,8 @@
 import warnings
 
 from pvlib import atmosphere
-from pvlib.tools import datetime_to_djd, djd_to_datetime
+from pvlib.tools import datetime_to_djd, djd_to_datetime, _optional_import
+ephem = _optional_import('ephem', 'PyEphem must be installed')
 
 
 NS_PER_HR = 1.e9 * 3600.  # nanoseconds per hour
@@ -487,7 +488,6 @@ def _ephem_to_timezone(date, tzinfo):
 
 def _ephem_setup(latitude, longitude, altitude, pressure, temperature,
                  horizon):
-    import ephem
     # initialize a PyEphem observer
     obs = ephem.Observer()
     obs.lat = str(latitude)
@@ -544,11 +544,6 @@ def sun_rise_set_transit_ephem(times, latitude, longitude,
     pyephem
     """
 
-    try:
-        import ephem
-    except ImportError:
-        raise ImportError('PyEphem must be installed')
-
     # times must be localized
     if times.tz:
         tzinfo = times.tz
@@ -630,10 +625,6 @@ def pyephem(time, latitude, longitude, altitude=0, pressure=101325,
     """
 
     # Written by Will Holmgren (@wholmgren), University of Arizona, 2014
-    try:
-        import ephem
-    except ImportError:
-        raise ImportError('PyEphem must be installed')
 
     # if localized, convert to UTC. otherwise, assume UTC.
     try:
@@ -943,8 +934,6 @@ def pyephem_earthsun_distance(time):
     pd.Series. Earth-sun distance in AU.
     """
 
-    import ephem
-
     sun = ephem.Sun()
     earthsun = []
     for thetime in time:

pvlib/tools.py

@@ -6,6 +6,7 @@
 import numpy as np
 import pandas as pd
 import pytz
+import importlib
 
 
 def cosd(angle):
@@ -344,3 +345,39 @@ def _golden_sect_DataFrame(params, VL, VH, func):
             raise Exception("EXCEPTION:iterations exceeded maximum (50)")
 
     return func(df, 'V1'), df['V1']
+
+
+def _optional_import(module_name, message):
+    """
+    Import a module, deferring import errors.
+
+    If the module cannot be imported, don't raise an error, but instead return
+    a dummy object that raises an error when the module actually gets used
+    for something.
+
+    Parameters
+    ----------
+    module_name: str
+        Name of the module to import, e.g. 'pandas'
+    message: str
+        Deferred error message, e.g. 'pandas must be installed for read_csv'
+    """
+    try:
+        return importlib.import_module(module_name)
+    except ImportError:
+        return _DeferredImportError(message)
+
+
+class _DeferredImportError:
+    """
+    Defer import errors until an imported package actually gets used.
+
+    Useful for importing optional dependencies at the top of a file
+    instead of hiding them inside the functions that use them.
+    """
+
+    def __init__(self, message):
+        self.message = message
+
+    def __getattr__(self, attrname):
+        raise ImportError(self.message)