Merge pull request #2722 from pbugnion/avoid-unnecessary-copies-in-file-upload-docs

Avoid unnecessary copies in snippets for file upload

Author: pbugnion

docs/source/examples/Widget List.ipynb

@@ -1054,7 +1054,7 @@
     "#=> ]\n",
     "```\n",
     "\n",
-    "The contents of the file uploaded are in the value of the `content` key. They are a memory view:\n",
+    "The contents of the file uploaded are in the value of the `content` key. They are a [memory view](https://docs.python.org/3/library/stdtypes.html#memory-views):\n",
     "\n",
     "```python\n",
     "[uploaded_file] = uploader.value\n",
@@ -1069,10 +1069,11 @@
     "#=> b'This is the content of example.txt.\\n'\n",
     "```\n",
     "\n",
-    "To get the contents as a string:\n",
+    "If the file is a text file, you can get the contents as a string by [decoding it](https://docs.python.org/3/library/codecs.html):\n",
     "\n",
     "```python\n",
-    "uploaded_file[\"content\"].tobytes().decode(\"utf-8\")\n",
+    "import codecs\n",
+    "codecs.decode(uploaded_file[\"content\"], encoding=\"utf-8\")\n",
     "#=> 'This is the content of example.txt.\\n'\n",
     "```\n",
     "\n",
@@ -1083,10 +1084,18 @@
     "    fp.write(uploaded_file[\"content\"])\n",
     "```\n",
     "\n",
-    "To convert the uploaded file into a Pandas dataframe, you need to convert it to a StringIO object:\n",
+    "To convert the uploaded file into a Pandas dataframe, you can use a [BytesIO object](https://docs.python.org/3/library/io.html#binary-i-o):\n",
     "\n",
     "```python\n",
-    "pd.read_csv(io.StringIO(uploaded_file[\"content\"].tobytes().decode(\"utf-8\")))\n",
+    "import io\n",
+    "import pandas as pd\n",
+    "pd.read_csv(io.BytesIO(uploaded_file[\"content\"]))\n",
+    "```\n",
+    "\n",
+    "If the uploaded file is an image, you can visualize it with an [image](#Image) widget:\n",
+    "\n",
+    "```python\n",
+    "widgets.Image(value=uploaded_file[\"content\"].tobytes())\n",
     "```\n",
     "\n",
     "<div class=\"alert alert-info\">\n",