changed ValueError to ImportError

[leo-gan]

changed ValueError to ImportError

Code cleaning.
Fixed inconsistencies in ImportError handling. Sometimes it raises ImportError and sometime ValueError.
I've changed all cases to the raise ImportError
Also:

• added installation instruction in the error message, where it missed;
• fixed several installation instructions in the error message;
• fixed several error handling in regards to the ImportError

Who can review?

@hwchase17
@dev2049

[dev2049]

bit hard to tell exactly what's going on here but guessing you just took all the downstream code out of the try/except without any functional changes?

[leo-gan]

yes, exactly.
BTW I've found several places where import is inside class/function but without try: except ImportError brackets. Is it OK, or would we add these brackets to all internal imports?

[dev2049]

i think some classes perform the check at instantiation, so in those cases it's fine not to check within an instance method (since you would've had to successfully instantiate the object to get there)

[leo-gan]

But then why this import is inside the code not at the file beginning? I'm still not comfortable with this idea. But the worst scenario we still get the same ImportError but without a clear explanation of how the missed package should be installed ("... use pip install <package-name>"). It is not a big deal.