In this paper, we characterize the MacWilliams extension property (MEP) and constant weight codes with respect to $ω$-weight defined on $\mathbb{F}^Ω$ via an elementary approach, where $\mathbb{F}$ is a finite field, $Ω$ is a finite set, and $ω:Ω\longrightarrow\mathbb{R}^{+}$ is a weight function. Our approach relies solely on elementary linear algebra and two key identities for $ω$-weight of subspaces derived from a double-counting argument. When $ω$ is the constant $1$ map, our results recover two well-known results for Hamming metric code: (1) any Hamming weight preserving map between linear codes extends to a Hamming weight isometry of the entire ambient space; and (2) any constant weight Hamming metric code is a repetition of the dual of Hamming code.