MNT: make certifi actually optional

tacaswell · tacaswell · commit 188338007fa5 · 2020-10-06T10:29:13.000-04:00
In matplotlib#18636 we removed certifi as a runtime dependency, but it turns out we do try to use it if you read from a url in a handful of places. This makes us fail gracefully and log if we can not import certifi.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -717,7 +717,11 @@ def is_url(filename):
 
 @functools.lru_cache()
 def _get_ssl_context():
-    import certifi
+    try:
+        import certifi
+    except ImportError:
+        _log.debug("Could not import certifi.")
+        return None
     import ssl
     return ssl.create_default_context(cafile=certifi.where())
 
@@ -726,7 +730,12 @@ def _get_ssl_context():
 def _open_file_or_url(fname):
     if not isinstance(fname, Path) and is_url(fname):
         import urllib.request
-        with urllib.request.urlopen(fname, context=_get_ssl_context()) as f:
+        ssl_ctx = _get_ssl_context()
+        if ssl_ctx is None:
+            _log.debug(
+                "Could not get certifi ssl context, https may not work."
+            )
+        with urllib.request.urlopen(fname, context=ssl_ctx) as f:
             yield (line.decode('utf-8') for line in f)
     else:
         fname = os.path.expanduser(fname)
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -1478,8 +1478,12 @@ def imread(fname, format=None):
         if len(parsed.scheme) > 1:  # Pillow doesn't handle URLs directly.
             # hide imports to speed initial import on systems with slow linkers
             from urllib import request
-            with request.urlopen(fname,
-                                 context=mpl._get_ssl_context()) as response:
+            ssl_ctx = mpl._get_ssl_context()
+            if ssl_ctx is None:
+                _log.debug(
+                    "Could not get certifi ssl context, https may not work."
+                )
+            with request.urlopen(fname, context=ssl_ctx) as response:
                 import io
                 try:
                     response.seek(0)
