How to get the list of user defined variables

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That's a good question. You can inspect the global variables that have been defined at any given point in a session, but most of those will be defined when various modules load on Sage startup. You can look just at those globals whose type is sage.symbolic.expression.Expression, there aren't too many at startup:

sage: G = globals()
sage: for k in G:
....:     if type(G[k]) == sage.symbolic.expression.Expression:
....:         print k
golden_ratio
log2
NaN
merten
I
twinprime
pi
catalan
brun
euler_gamma
x
khinchin
i
glaisher
mertens

Anything not on that list is a global symbolic expression that is defined in your loaded session. Variables will be among these.