Most code that is vanilla Python will work just fine with PyPy. However, in real life, your code most likely also uses (imports) other Python libraries, you just need to make sure they work OK with PyPy.
Many, many, many libraries already work fine with PyPy, so give it a shot.
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Here is the dumbest question in the world: our application has a gui which is pyqt, can we use pypy? Aside from pyqt, its completely vanilla python.
Most code that is vanilla Python will work just fine with PyPy. However, in real life, your code most likely also uses (imports) other Python libraries, you just need to make sure they work OK with PyPy.
Many, many, many libraries already work fine with PyPy, so give it a shot.