python: SyntaxError: EOL while scanning string literal

(Assuming you don't have/want line breaks in your string...)

How long is this string really?

I suspect there is a limit to how long a line read from a file or from the commandline can be, and because the end of the line gets choped off the parser sees something like s1="some very long string.......... (without an ending ") and thus throws a parsing error?

You can split long lines up in multiple lines by escaping linebreaks in your source like this:

s1="some very long string.....\
...\
...."

You are not putting a " before the end of the line.

Use """ if you want to do this:

""" a very long string ...... 
....that can span multiple lines
"""

I had this problem - I eventually worked out that the reason was that I'd included \ characters in the string. If you have any of these, "escape" them with \\ and it should work fine.