A constitutional government by definition is a

Gives definitions of 'constitutionalism' and 'constitutional government' emphasizing restraints on government and limited (not arbitrary) government.

A student could use this rule to argue that constitutional government focuses on limits and checks rather than simple rule by one organ (the legislature), so one should check whether the legislature alone provides such restraints.

Defines constitution as rules determining organization of government and distribution of powers among organs.

Combine with a map of institutional powers to see whether the legislature is the sole source of authority under a constitution or one among several organs.

States the separation of powers: legislature makes laws, executive implements them, judiciary checks conformity with the Constitution.

Use this separation to infer that in systems observing constitutionalism, government is shared among organs, so 'government by the legislature' would conflict with this principle unless other organs are weak.

Explains parliamentary system where the executive is responsible to the legislature (executive originates from legislature).

A student could note that parliamentary government creates close legislature-executive links—this might make the government appear 'by the legislature' in practice, so check whether this is a feature of a particular constitutional system (e.g., Westminster) rather than a general definition.

Says the Constitution defines authority and limits of the three organs, and Parliament must operate within constitutional limits; distinguishes legislative authority from constituent authority.

One can test whether 'government by the legislature' fits by examining if the legislature's authority is supreme or constrained by constitutional rules and other institutions (e.g., judiciary, constituent procedures).