Add Features to check the environment at runtime

Eventually this should replace is_package_installed()
checks which are annoying for packagers.

src/sage/databases/cremona.py

@@ -53,7 +53,6 @@
 
 import sage.schemes.elliptic_curves.constructor as elliptic
 from .sql_db import SQLDatabase, verify_column
-from sage.misc.package import is_package_installed
 from sage.env import SAGE_SHARE
 from sage.misc.all import walltime
 
@@ -606,24 +605,25 @@ def __init__(self, name, read_only=True, build=False):
             sage: c.name
             'cremona mini'
         """
+        if name is None:
+            raise ValueError("name must not be None.")
+        if build and read_only:
+            raise ValueError("only one of build and read_only can be set at once.")
+
         self.name = name
-        name = name.replace(' ','_')
-        db_path = os.path.join(SAGE_SHARE, 'cremona', name+'.db')
+
         if build:
-            if name is None:
-                raise RuntimeError('The database must have a name.')
-            if read_only:
-                raise RuntimeError('The database must not be read_only.')
+            db_path = os.path.join(SAGE_SHARE, 'cremona', name.replace(' ','_')+'.db')
             SQLDatabase.__init__(self, db_path, read_only=read_only, \
                     skeleton=_miniCremonaSkeleton)
-            return
-        if not os.path.isfile(db_path):
-            raise ValueError("Desired database (='%s') does not "%self.name \
-                    + "exist")
-        SQLDatabase.__init__(self, db_path, read_only=read_only)
-        if self.get_skeleton() != _miniCremonaSkeleton:
-            raise RuntimeError('Database at %s does '%(self.__dblocation__) \
-              + 'not appear to be a valid SQL Cremona database.')
+        else:
+            from sage.misc.feature_test import DatabaseCremona
+            database = DatabaseCremona(name)
+            database.require()
+            SQLDatabase.__init__(self, database.absolute_path(), read_only=read_only)
+            if self.get_skeleton() != _miniCremonaSkeleton:
+                raise RuntimeError('Database at %s does '%(self.__dblocation__) \
+                  + 'not appear to be a valid SQL Cremona database.')
 
     def __iter__(self):
         """
@@ -824,14 +824,15 @@ def coefficients_and_data(self, label):
             if N < self.largest_conductor():
                 message = "There is no elliptic curve with label " + label \
                     + " in the database"
-            elif is_package_installed('database_cremona_ellcurve'):
+            from sage.misc.feature_test import DatabaseCremona
+            cremona_database_presence = DatabaseCremona().is_present()
+            if cremona_database_presence:
                 message = "There is no elliptic curve with label " + label \
                     + " in the currently available databases"
             else:
                 message = "There is no elliptic curve with label " \
-                    + label + " in the default database; try installing " \
-                    + "the optional package database_cremona_ellcurve which " \
-                    + "contains the complete Cremona database"
+                    + label + " in the default database. " \
+                    + cremona_database_presence.resolution
             raise ValueError(message)
         ainvs = eval(c[0])
         data = {'cremona_label': label,
@@ -1418,24 +1419,25 @@ def __init__(self, name, read_only=True, build=False):
             sage: c.name                            # optional - database_cremona_ellcurve
             'cremona'
         """
+        if name is None:
+            raise ValueError("name must not be None.")
+        if build and read_only:
+            raise ValueError("only one of build and read_only can be set at once.")
+
         self.name = name
-        name = name.replace(' ','_')
-        db_path = os.path.join(SAGE_SHARE, 'cremona', name+'.db')
+
         if build:
-            if name is None:
-                raise RuntimeError('The database must have a name.')
-            if read_only:
-                raise RuntimeError('The database must not be read_only.')
+            db_path = os.path.join(SAGE_SHARE, 'cremona', name.replace(' ','_')+'.db')
             SQLDatabase.__init__(self, db_path, read_only=read_only, \
-                    skeleton=_cremonaSkeleton)
-            return
-        if not os.path.isfile(db_path):
-            raise ValueError("Desired database (='%s') does not "%self.name \
-                    + "exist")
-        SQLDatabase.__init__(self, db_path, read_only=read_only)
-        if self.get_skeleton() != _cremonaSkeleton:
-            raise RuntimeError('Database at %s does '%(self.__dblocation__) \
-              + 'not appear to be a valid SQL Cremona database.')
+                    skeleton=_miniCremonaSkeleton)
+        else:
+            from sage.misc.feature_test import DatabaseCremona
+            database = DatabaseCremona(name)
+            database.require()
+            SQLDatabase.__init__(self, database.absolute_path(), read_only=read_only)
+            if self.get_skeleton() != _cremonaSkeleton:
+                raise RuntimeError('Database at %s does '%(self.__dblocation__) \
+                  + 'not appear to be a valid SQL Cremona database.')
 
     def allbsd(self, N):
         r"""
@@ -1673,7 +1675,8 @@ def CremonaDatabase(name=None,mini=None,set_global=None):
     if name is None and not set_global:
         return _db
     if set_global and name is None:
-        if is_package_installed('database_cremona_ellcurve'):
+        from sage.misc.feature_test import DatabaseCremona
+        if DatabaseCremona().is_present():
             name = 'cremona'
         else:
             name = 'cremona mini'