You can break these rules if you can talk your pair into agreeing with you.
SANDI: I’ve been wanting to give — okay, so people wanted rules for me like when I go out and see strangers when I look at their code. So, I finally gave them some rules and the rules are, and this is painting a big target on me to give you these rules. But I want to say them to you and see what you guys think about them.
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SANDI: Okay. So, I ended up giving, “This is the bottom line. Your class can be no longer than 100 lines of code. Your methods can be no longer than five lines of code.” I wanted four but I felt like I had to give them five. “You can pass no more than four parameters and you can’t just make it one big hash. Your controller, in a Rails app, when a call comes into your controller, you can only instantiate one object to do whatever it is that needs to be done.”
SANDI: “And that your view can only know about one instance variable.” So those are the rules I gave them. I told them they could break them, right? They could break them.
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CHUCK: So, one thing that really strikes me with these rules is that for the most part, if you really try, you should be able to follow them. And if you can’t follow them — see, this is the thing with programming rules that I think people don’t understand is that when you say, “I’m giving you this rule,” they think, “Oh well, I’m sure I can find an instance that breaks this rule.” The whole point is yeah, but when you break the rule, you should be able to explain exactly why you need to break the rule. In that way, then you can justify what you’re doing. But otherwise, you’re not forced to think about what you’re coding and that’s really what the issue is.
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SANDI: I told them they could break the rules if they could talk their pair into agreeing with them.
Source: http://rubyrogues.com/087-rr-book-clubpractical-object-oriented-design-in-ruby-with-sandi-metz/