Control structures

In Jinja2, a control structure refers to the statements in a template that control the flow of the engine parsing the template. These structures include conditionals, loops, and macros. Within Jinja2 (assuming the defaults are in use), a control structure will appear inside blocks of {% ... %}. These opening and closing blocks alert the Jinja2 parser that a control statement has been provided instead of a normal string or variable name.

Conditionals

A conditional within a template creates a decision path. The engine will consider the conditional and choose from two or more potential blocks of code. There is always a minimum of two: a path if the conditional is met (evaluated as true), and either an explicitly defined else path if the conditional is not met (evaluated as false) or, alternatively, an implied else path consisting of an empty block.

The statement for a conditional is the if statement. This...