Is the reason neither Yoda or Obi-Wan thought it a good idea to inform Luke that
his father and Vader were the same person
ever discussed? Were they hoping that
Vader himself
didn't realize this, and that it wouldn't come up? Or was there an explicit rationale behind hiding this fact from Luke?
Wouldn't it seem risky to send Luke in blind, with the chance of him finding out without their support, considering how emotional attachment can be a door into the Dark Side?
If either Obi-Wan or Yoda had told Luke the truth, then Luke would likely have sought out Vader at the first opportunity.
In A New Hope, let's suppose the conversation with Obi-Wan on Tatooine went like this instead:
OBI-WAN: This lightsaber belonged to your father, Lord Vader.
LUKE: Wait. Back up. My father is who?
OBI-WAN: Darth Vader.
LUKE: Say what?? That creepy evil guy? You're joking!
OBI-WAN: Nope. Dead serious. The infamous Darth Vader who has captured your holographic princess...he's your dad. For realz.
C3PO: Oh dear...
LUKE: You're wrecking my head. This is insane!
OBI-WAN: Another thing: that princess is your sister. Let's go save her, shall we?
Later, on the Death Star:
LUKE: Dad, why did you kill Obi-Wan?! He was my friend!!
VADER: Ah, my long lost son. Listen, Obi-Wan deserved it. Come, let me tell you a tale about a place called Mustafar...
At that point, either Luke ends up on the Dark Side or dead, depending on how the father-son bonding goes.
The point is, had he known the truth, Luke would have sought Vader out too early, before his training was complete, and this would have only had negative consequences for Luke and the galaxy-at-large.
This is supported by what Yoda says to Luke in Return of the Jedi just prior to expiring:
YODA: Your father, he is. Told you, did he?
LUKE: Yes.
YODA: Unexpected, this is, and unfortunate.
LUKE: Unfortunate that I know the truth?
YODA: No. Unfortunate that you rushed to face him. That incomplete, was your training. That not ready for the burden, were you.
In other words, they would have told him eventually, but at a suitable time, when they could be more certain that he had the training and maturity to avoid his father's fate.
The previous answer was most excellent. Full of wisdom, it was. :)
However, another answer is simply based on what I call 'the necessities of story-telling' - I don't know the regular term for this but here it is:
If Luke is informed as to his father being the great bad Lord Vader, then the plot as it is, and the revelation at a dramatic time, etc. is derailed, in any one of various ways (as the previous answer has already explained).
Also, quite simply, Luke might answer such an early revelation with:
If he's my Dad, then why don't we go seek help from him, since I'm his son?
[Responses:]
Shut up Luke. You should really know better, by now.
Mess with plot, will you? consequences there are, author might kill you off.
[Oh deah, that would be so against all the proper protocols to be used when addressing a nobleman of the Empire. He would surely punish you horribly in some way, to emphasize his rank, and his right to be free of importunities from what he might term mere beggar-boys!]
or Luke might just say:
This must be a really bad man, to have neglected and abandoned me for so long. This confirms that we must oppose him completely and in every way.
[....and, the drama is made flatter by that much. No revelation, no conflicted emotions, no upsets, just at most, "What a bad 'Father' you were! take that!"]
In sum, revealing it too early would be unhelpful to the story. And, the main plot elements of the ('early' i.e. Ep. IV-VI) Star Wars movies are somewhat dependent on reversals of the usual:
the hero's father is the main villain
the Princess is the hero's sister
the wise elder throws his life away for obscure reasons (-perhaps I'm wrong here. Sorry.)
the plot is all over the place - many places, many planets
and so forth. Having (1) and (2) known to all members of the plot early on, would take that much away.
There does not appear to be a canon answer. However, one may extrapolate from the way the Jedi approach problems. Luke is constant being advised to feel the Force. These feelings can be easily clouded by thoughts (many characters are advised to "empty their mind" when they are having trouble using the force). It would be reasonable to assume Yoda and Obi Wan felt such a significant piece of knowledge would cloud Luke's ability to feel the Force around him, making him weak. And, as Praxis pointed out, such knowledge is not one that one empties from their mind easily.
Besides, it's all lies anyway! We all know Obi Wan killed his father until the magic of post production changed our reality forever :)
They wanted to keep Vader from finding out, remember the children were hidden from him. Luke was not yet able to control the force or his emotions so if (and when) he broadcast the information through the force and Vader would (and did) identify them.
Luke knowing was not the issue, Vader knowing was the issue.
They, themselves, didn't know how the Force was going to be balanced, so they could not interfere in such a way. The Force must be balanced in both its dark aspects and its light aspects. You'll note that Yoda respects it.
Once Luke had referred to Ben by his Jedi name (Obi-Wan), Luke was seen as a sort of "chosen One". You can see it by the look on the old man's face. After that, Obi-Wan's journey was also part of that balance (Annakin/Vader was his apprentice) and could not interfere on the question.
