Assuming you will have something before you actually have it; similar to "counting your chickens before they hatch" (which is commonly attributed to Aesop).
"The Mariners could do better than expected this year."

"Let's not kid ourselves. Check your premises before you make a prediction."

"I'm assuming a number of things go right. Maybe I'm just counting my monkeys before they're out of the trees."
by Rusanova February 3, 2012
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