rough basement plumbing

I have been reading many posts in this forum and seeing that you must have a P-trap in your shower drain pipe. Is this code? I have a basement bathroom and am starting to finish and everything is roughed in, if this is so important then why would a plumber not put it in when the rough plumbing was done?

p-trap

poure water down the shower drain and you should see water.

What your saying is that if there is a p-trap and I pour water down the drain some water will stay in the trap?

YUP! that's the idea...

The P-trap is code required... not to mention a good idea..... it keeps sewer gases from wafting up your drain into your home.........

Not to drag this out forever, but a P trap is what keeps sewer gas from entering your home. Every drain must have one. The trap is built into your toilet(s), some, like the basement shower, may be covered with concrete, others like you sink, are exposed and easily disassembled to retreive rings and clear small clogs. They work by holding water in loop which blocks the gas. If you have a drain that is seldom used, the water could evaporate, so periodically you should run a little water to recharge the trap. If your drain in question is not trapped or if the trap is dry, you would likely smell sewer gas. If it was properly roughed-in, it should be trapped and then the stub capped.

Many times there will be a 'holdout box' for installation of the trap after concrete is poured. This is to allow proper/necessary positioning of the trap to the shower.