Question

I’m having a problem by trying to assign a lambda as a callback to optparse to handle the case where multiple -v’s are passed to the app, which should increment the global variable DEBUG each time a -v is seen.

# globals
DEBUG = 0
(...)
def main():
[...]
    parser.add_option("-v",  action="callback",
        callback=lambda a,b,c,d: DEBUG += 1
        help="Verbose (-vvv for added verboseness)")

When this gets executed, the result is:

    callback=lambda a,b,c,d: DEBUG += 1
                                    ^
SyntaxError: invalid syntax

Any idea or alternative (clean) suggestion?

Answer 1

A lambda has to be an expression, while assignment like that is a statement. You will probably have to do it like this:

# globals
DEBUG = 0
(...)
def main():
[...]
    def increment(*args):
        global DEBUG
        DEBUG += 1

    parser.add_option("-v",  action="callback",
        callback=increment,
        help="Verbose (-vvv for added verboseness)")