add Base.hasproperty?

[stevengj]

I just realized that we don't have a hasproperty(x, s) function, analogous to haskey. We could define one with the default definition:

hasproperty(x, s::Symbol) = s in propertynames(x)

This came up in https://github.com/JuliaPy/PyCall.jl/pull/517/files#r212317623

[ararslan]

Relatedly, a hasfield function to accompany getfield/setfield! may be nice. This definition actually works for that:

hasfield(T::DataType, name::Symbol) = fieldindex(T, name, false) > 0

[vtjnash]

Agreed. I think the fallback may be isdefined though.