Actual value missing in error message

[potmo]

The error message for a spec can be like this
Invalid "sessionid" property value, expected string

It would be very nice to see the actual value like
Invalid "sessionid" property value, expected string. Was: 1234

[potmo]

A subset of that is errors like:
Invalid anObject.withAProperty property value, expected specified by dataspec
would be a lot nicer to get

Invalid basket.fruit property value, expected "banana" but was "coconut"

[dimsmol]

Actually, it's by design.

There are two reasons for it:

• The wrong datum itself can be unexpectedly large structure (like a dictionary with dictionaries and arrays inside) that we don't want to pass back to caller in case if we are validating HTTP request or something similar
• The validator itself can be very complex, including not() constructions or sw(), so it may be complicated to formulate correctly what is actually expected. Even much simpler cases like v(num, range(1,5)) are not so simple according to this.

So, it was decided to make message simple, assuming it is always easy to take a look at the data provided and validators applied to figure out the problem.

But I agree, for most of simple cases (and most validation cases are really simple just like in your examples) a better solution can be provided.

Cannot promise to fix it in nearest future, but I will definitely return to this issue some day.