Revert PR #3.

This reverts commit da86036.

_episodes/02-files.md

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+---
+title: "Opening files"
+teaching: 20
+exercises: 0
+questions:
+ - "How do I open a ROOT file with Uproot?"
+ - "How can I tell what's in the file?"
+ - "How do I access the contents?"
+objectives:
+ - "Open a ROOT file and list its contents."
+ - "Access objects within a ROOT file."
+keypoints:
+ - "Opening and navigating files in Uproot isn't that different from doing so in ROOT."
+ - "Use `.keys()` to see the contents of a file."
+ - "Use the form `file['key']` to access an object inside a file."
+---
+
+# Example motivation
+
+In order to learn how to use Uproot, we'll try to do a very short and simple analysis to look for resonances in a dimuon events.
+The example ROOT file is from real CMS data during proton-proton collisions in 2012.
+Make sure you've downloaded it from the [Setup]({{ page.root }}/setup.html) page and put it in your working directory.
+Our first goal is just to get to the data within Uproot, so we need to open the file and navigate to the muon information.
+
+# Opening a file
+
+The first thing you must do whenever you want to use Uproot is import it, just like any other Python module:
+
+~~~
+import uproot
+~~~
+{: .language-python}
+
+> ## Import errors?
+>
+> If you get something like:
+> 
+> ~~~
+> Traceback (most recent call last):
+>   File "<stdin>", line 1, in <module>
+> ModuleNotFoundError: No module named 'uproot'
+> ~~~
+> {: .output}
+> 
+> then this means Uproot hasn't been installed properly.
+> Check the [Setup]({{ page.root }}/setup.html) page for detailed instructions.
+> If you're using `conda` (like Anaconda or Miniconda),
+> make sure you've activated the same environment where you installed Uproot.
+{: .callout}
+
+Now open the ROOT file and assign it to a variable (which I've named `file` here):
+
+~~~
+file = uproot.open('uproot-tutorial-file.root')
+~~~
+{: .language-python}
+
+If you inspect `file`, you can see that it's a `ReadOnlyDirectory` object:
+
+~~~
+file
+~~~
+{: .language-python}
+~~~
+<ReadOnlyDirectory '/' at 0x(some hexadecimal number here)>
+~~~
+{: .output}
+
+# File contents
+
+Just like any other kind of directory, you can list the contents (of the file).
+The name of each item in the file is called a "key".
+
+~~~
+file.keys()
+~~~
+{: .language-python}
+~~~
+['Events;1']
+~~~
+{: .output}
+
+We can see that there is one key: "Events".
+This doesn't tell us what kind of object it refers to, though.
+ROOT files can contain many different types of objects, including subdirectories.
+The following function provides a way to inspect the types of each item:
+
+~~~
+file.classnames()
+~~~
+{: .language-python}
+~~~
+{'Events;1': 'TTree'}
+~~~
+{: .output}
+
+The output contains pairs of the form `name: type`.
+Therefore the key `Events` refers to a TTree object.
+This is where all the data in this file is stored.
+
+> ## Why the `;1`?
+>
+> You may be wondering why there's a `;1` after "Events".
+> This notation refers to the *cycle number*, which is a detail of the ROOT file format that we don't need to care about.
+> This `;1` shows up if you open the file in ROOT itself as well.
+{: .callout}
+
+# Accessing contents
+
+Now we want to actually access the object inside the file.
+You can do this just as you would to get an item in an array:
+
+~~~
+file['Events']
+~~~
+{: .language-python}
+~~~
+<TTree 'Events' (6 branches) at 0x(hexadecimal number)>
+~~~
+{: .output}
+
+This expression refers to the actual TTree object, which we will look at next.
+
+{% include links.md %}