csv_metadata_quality/check.py: requests cache

Allow overriding the directory for the requests cache. In the case
of csv-metadata-quality-web, which currently runs on Google's App
Engine, we can only write to /tmp.

csv_metadata_quality/check.py

@@ -1,3 +1,4 @@
+import os
 import re
 from datetime import datetime, timedelta
 
@@ -242,7 +243,11 @@ def agrovoc(field, field_name):
 
     # enable transparent request cache with thirty days expiry
     expire_after = timedelta(days=30)
-    requests_cache.install_cache("agrovoc-response-cache", expire_after=expire_after)
+    # Allow overriding the location of the requests cache, just in case we are
+    # running in an environment where we can't write to the current working di-
+    # rectory (for example from csv-metadata-quality-web).
+    REQUESTS_CACHE_DIR = os.environ.get("REQUESTS_CACHE_DIR", ".")
+    requests_cache.install_cache(f"{REQUESTS_CACHE_DIR}/agrovoc-response-cache", expire_after=expire_after)
 
     # prune old cache entries
     requests_cache.core.remove_expired_responses()