Let $k$ be any field and $G$ a semi-abelian variety over $k$, i.e., an algebraic group that fits into an exact sequence


$$ 1 \to T \to G \to A \to 1$$


of algebraic groups, where $T$ is an algebraic torus and $A$ is an abelian variety. I have heard somewhere that, given an algebraic group $G$, if $G$ is semi-abelian then it is so in a unique way, meaning that $T$, $A$ and even the maps in the short exact sequence are uniquely determined by $G$. But I have failed to find a reference for this in full generality (or counterexamples). 



Uniqueness of presentation for semi-abelian varieties
First of all, there are no non-trivial homomorphisms from a torus $T$ to an abelian variety $A$ (also true from additive group $\mathbb{G}\_a$ to $A$, or other unipotent like Witt groups). This is because there are no non-constant rational maps from $\mathbb{A}^1$ to $A$ (see for example [Milne's book on abelian varieties](https://www.jmilne.org/math/CourseNotes/AV.pdf), proposition I.3.9). 


Hence, any homomorphism $f$ from a semiabelian variety $$ 0\to T \to G \to A \to 0$$ to another semiabelian variety $$ 0\to T' \to G' \to A' \to 0$$ gives homomorphisms $g:T\to T'$ and $h:A\to A'$. The homomorphism $f$ is an isomorphism if and only if both $h$ and $g$ are. 


