Issue a warning and raise a more useful exception if someone tries to call Tag.select() without SoupSieve installed.

2 files changed, 27 insertions, 2 deletions

diff --git a/NEWS.txt b/NEWS.txt
index 6e44504..4f5278a 100644
--- a/NEWS.txt
+++ b/NEWS.txt
@@ -1,5 +1,19 @@
 = 4.7.0 (Unreleased)
 
+* Beautiful Soup's CSS Selector implementation has been replaced by a
+  dependency on Isaac Muse's SoupSieve project (the soupsieve package
+  on PyPI). The good news is that SoupSieve has a much more robust and
+  complete implementation of CSS selectors, resolving a large number
+  of longstanding issues. The bad news is that from this point onward,
+  SoupSieve must be installed if you want to use the select() method.
+
+  You don't have to change anything lf you installed Beautiful Soup
+  through pip (SoupSieve will be automatically installed when you
+  upgrade Beautiful Soup) or if you don't use CSS selectors from
+  within Beautiful Soup.
+
+  SoupSieve documentation: https://facelessuser.github.io/soupsieve/
+
 * Fix a number of problems with the tree builder that caused
   trees that were superficially okay, but which fell apart when bits
   were extracted. Patch by Isaac Muse. [bug=1782928]
diff --git a/bs4/element.py b/bs4/element.py
index 7734f80..97403c9 100644
--- a/bs4/element.py
+++ b/bs4/element.py
@@ -9,7 +9,14 @@ except ImportError , e:
 import re
 import sys
 import warnings
-import soupsieve
+try:
+    import soupsieve
+except ImportError, e:
+    soupsieve = None
+    warnings.warn(
+        'The soupsieve package is not installed. CSS selectors cannot be used.'
+    )
+
 from bs4.dammit import EntitySubstitution
 
 DEFAULT_OUTPUT_ENCODING = "utf-8"
@@ -1347,7 +1354,11 @@ class Tag(PageElement):
         
         if limit is None:
             limit = 0
-
+        if soupsieve is None:
+            raise NotImplementedError(
+                "Cannot execute CSS selectors because the soupsieve package is not installed."
+            )
+            
         return soupsieve.select(selector, self, namespaces, limit, **kwargs)
 
     # Old names for backwards compatibility