inject_misc_tags no longer fails if document has no head. Resolves #16.

src/caterpillar.py

Index: src/caterpillar.py
diff --git a/src/caterpillar.py b/src/caterpillar.py
index 4a0a8b5c38e4e035c8b1953a6cd4babbfe734961..c658708a54f6d69c159f8f93f07db947b8d68e92 100755
--- a/src/caterpillar.py
+++ b/src/caterpillar.py
@@ -286,6 +286,13 @@ def inject_misc_tags(soup, ca_manifest, root_path, html_path):
       This can be either absolute or relative.
     html_path: Path to the HTML document being modified.
   """
+  if not soup.head:

[Matt Giuca, 2016/01/29 02:19:26]

head = soup.head
if not head:
  ...

Now you are guaranteed to have a "head" variable and you can replace instances
of "soup.head" below with just "head".

[Matthew Alger, 2016/01/29 02:41:51]

Okay, done. I'm not sure I see the benefit in this.

[Matt Giuca, 2016/01/29 03:04:13]

Mostly performance.

You generally want to avoid doing the same operation multiple times (and instead
storing it in a variable). Whether that's computations or just lookups, the same
principle applies (but it's less of a problem for lookups).

In Python (unlike static languages), member lookups are hash table lookups so
they are not free. And I don't know for sure, but I'd guess that this particular
lookup (in BeautifulSoup) may actually be an HTML tree search rather than a
simple hash lookup.

(Note: You should generally avoid micro-optimizations unless you have a pressing
performance need. But this sort of thing is more of a matter of good style/habit
than us specifically needing performance in this part of the program.)

+    head = soup.new_tag('head')
+    if soup.html:
+      soup.html.insert(0, head)
+    else:
+      soup.insert(0, head)
+
   # Add manifest link tag.
   manifest_path = os.path.join(root_path, PWA_MANIFEST_FILENAME)
   manifest_link = soup.new_tag('link', rel='manifest', href=manifest_path)