fix: don't require ijson if @decorators.cache_iterator is not used

.travis.yml

@@ -1,4 +1,5 @@
-dist: xenial
+# do not use xenial, or it will fail pages deploy
+# https://travis-ci.community/t/failed-to-deploy-because-of-ruby-error/1143
 sudo: required
 language: python
 python:
@@ -27,14 +28,6 @@ jobs:
     script: make html
 
     deploy:
-      - provider: pages
-        skip-cleanup: true
-        github-token: $GH_TOKEN
-        keep-history: true
-        on:
-          branch: master
-        local-dir: docs/build/html
-
       - provider: script
         skip_cleanup: true
         on:
@@ -48,5 +41,13 @@ jobs:
           tags: true
         file: dist/*
 
+      - provider: pages
+        skip-cleanup: true
+        github-token: $GH_TOKEN
+        keep-history: true
+        on:
+          branch: master
+        local-dir: docs/build/html
+
 after_failure:
   - pip freeze

stutils/decorators.py

@@ -1,7 +1,4 @@
 
-import ijson.backends.yajl2 as ijson
-import pandas as pd
-
 import json
 import logging
 import os
@@ -11,6 +8,7 @@
 import tempfile
 import threading
 
+import pandas as pd
 import stutils
 from stutils.sysutils import mkdir
 
@@ -191,7 +189,13 @@ class cache_iterator(fs_cache):
         in Python2, json instantiates loaded strings as unicode, so cached
             result might be slightly different from original
     """
+
     def __call__(self, func):
+        try:
+            import ijson.backends.yajl2 as ijson
+        except ImportError:
+            raise ImportError("Please install yajl-tools to use this decorator")
+
         @wraps(func)
         def wrapper(*args):
             cache_fpath = self.get_cache_fname(