What is the effect of section 1237.5 of the California Criminal Code on an appeal against a guilty plea?

We recognize that in People v. Breckenridge, supra, 5 Cal.App.4th at page 1101, 8 Cal.Rptr.2d 1, we stated that "[t]he purpose of section 1237.5 is to discourage and weed out frivolous or vexatious appeals following guilty pleas," and that "it is inappropriate and unwise to let expediency outweigh the legislative command that makes compliance with section 1237.5 a condition precedent to an appellate attack on the validity of a guilty plea."