Autocmd bufnewfile causing "Trailing characters" error on relative paths - vim

Question

I am having issue with my .vimrc file. I have completed an autocommand for all python and sh files. I have included both below. All works as expected when using a direct path ie:
gvim test.py
If I use a path relative to the cwd, however, such as:
gvim ../test.py
I receive the following error:
Error detected while processing BufNewFile Auto commands for "*.{py,sh}"
E488: Trailing characters
Any ideas on how to fix this problem?
autocmd bufnewfile *.{py,sh}
\ let path = expand("~/bin/Templates/")|
\ let extension = expand("%:e")|
\ let template = path . extension|
\ let name = "John Doe" |
\ if filereadable(template)|
\ execute "silent! 0r" . template|
\ execute "1," . 10 . "g/# File Name:.*/s//# File Name: " .expand("%")|
\ execute "1," . 10 . "g/# Creation Date:.*/s//# Creation Date: " .strftime("%b-%d-%Y")|
\ execute "1," . 10 . "g/Created By:.*/s//Created By: " . name|
\ execute "normal Gdd/CURSOR\<CR>dw"|
\ endif|
\ startinsert!
autocmd bufwritepre,filewritepre *.{py,sh}
\ execute "normal ma"|
\ execute "1," . 10 . "g/# Last Modified:.*/s/# Last Modified:.*/# Last Modified: "
\ .strftime("%b-%d-%Y")
autocmd bufwritepost,filewritepost *.{py,sh}
\ execute "normal 'a"
The template for python files is as follows:
#!/usr/bin/python
# File Name: <filename>
# Creation Date: <date>
# Last Modified: <N/A>
# Created By: <Name>
# Description: CURSOR

Answer 1

First of all, let's have a look at :help 10.2:
The general form of the `:substitute` command is as follows:
:[range]substitute/from/to/[flags]
Please keep /[flags] in mind. Now when you enter gvim test.py in the command line, the following command is executed in Vim:
:s//# File Name: test.py
But when you enter gvim ../test.py Vim executes:
:s//# File Name: ../test.py
so Vim uses test.py as :substitute's flags and that's not the desired behavior.
What you need is to replace expand("%") with expand("%:t") to get only the file name. See :help expand() for details.