Shell script bafflement

Question

I recently downloaded a shell script that runs an embedded python script.
The first few lines looks like this:
<pre>
#!/bin/sh
exec python $0 ${1+"$@"}
</pre>
I know what #!/bin/sh does, and I know what "exec python" does, but what do the
<pre>
</pre>
and
<pre>
$0 ${1+"$@"}
</pre>
mean???
(The actual python script follows these lines.) I assume the gist of it is that it runs sh, captures any command line arguments given after the name of the shell script executable, and then turns this all over to python as if the python code that follows had been invoked from the command line with those arguments following.
But I'd like to understand it on the level of the actual syntax....

Answer 1

Daniel's post explains almost everything; I just want to add some minor points (and one question).
For /bin/sh, the first line
is equivalent to
which is do-nothing built-in command of sh.
(If the first line is
or
then sh will try to find a command "" and results in 'command not found'.)
Then the second line is executed by sh, and the process is replaced by python. The third line
is illegal (or incomplete) as a shell script, but it's not a problem because sh does not read the line.
Now python reads the script from the beginning again.
In python, """ starts a multi-line string. The string starts just after """, so the string is
:"<newline>exec python $0 ${1+"$@"}
This is not a comment for python, but python ignores any simple expression in a script anyway. For example
#!/usr/bin/python
a = "foo"
a
"bar"
is a legal python script. The second and third lines (a and "bar") are simple expressions and do nothing.
One of the reason for using this trick is portability. The main problem is that python may be installed in /usr/bin, /usr/local/bin, /sw/bin, or anywhere else, depending on the system. So if a python script starts with
#!/usr/bin/python
then it does not run if python is installed in other places. A trick often used is
#!/usr/bin/env python
which will search python in the user's PATH. But this trick works only if /usr/bin/env exists.
Now a (minor) question:
What is the difference between ${1+"$@"} and "$@"? The former is more portable than the latter? If so, why?
/bin/sh may be a pure (true) Bourne-shell, bash or ksh (or any other?) depending on the system. I tried both bash and ksh, but ${1+"$@"} and "$@" seem to give the same results.
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