Comma Before Parenthesis

Parentheses are used to give additional information to the reader: information which is not required and which would disrupt the flow of the sentence if written as a non-restrictive clause. Commas may be placed after the closing parenthesis but not before either the opening or the closing parenthesis. If the sentence would not require any commas if the parentheses were removed, the sentence should not have any commas when the parentheses are added.

After opening the new cookie tin, (and eating several of the cookies), Mary had a hard time replacing the lid.

The comma after tin should be removed. The comma after the closing parenthesis is correctly used as it separates a dependent clause (after opening the new cookie tin) from an independent clause (Mary had a hard time replacing the lid).

After opening the new cookie tin (and eating several of the cookies,) Mary had a hard time replacing the lid.

After opening the new cookie tin (and eating several of the cookies), Mary had a hard time replacing the lid.

Peter cleaned his room, (grumbling all the while,) before going out to play basketball.

This sentence does not require any commas; both should be removed.