Thank you -- Of all the solutions offered so far on several threads, this is the first solution that has allowed me to open iPhoto since updating to 10.10.3.
I've owned each major iteration of the iMac since 1999. I bought this iMac 5 days ago -- with iPhoto, only to find it gone after running a routine software upgrade. I'm disappointed with the hubris of Apple, thinking it's OK to sell me a computer equipped with something I want, and taking that feature away immediately after my computer is set up. (Like ordering your usual dinner, and before you take your first bite, a waiter comes over, grabs your plate, and replaces it with something they just dreamed up in the kitchen, saying, "get used to it, we decided it's what you will like.")
Time will tell if Apple does the right thing here -- not the right thing for their shareholders, the right thing for their customers. My reasons for liking iPhoto are that iPhoto allows archiving my collection by keywords, and by events illustrated by a representative photo of that day's photos. I know that most professional photographers prefer Adobe Lightroom to any Apple products, but I always liked iPhoto for organizing photos from my camera, and then processing in Photoshop.
The idea of Photos saving just a cheesy small file on my computer and keeping the original on the cloud with a 5GB limit before fees kick in, can only appeal to Facebook denizens and Apple apologists (and of course, shareholders.)
Todd, thanks again for the workaround. I hope you will post this to the other threads dealing with this issue.