dynamic content for Flask dropdown menu in HTML form

[bhpayne]

In https://github.com/allofphysicsgraph/ui_v8_website_flask_neo4j/blob/gh-pages/webserver/pdg_app.py there's a class

class SpecifyNewExpressionForm(FlaskForm):
    expression_latex_lhs = StringField(label="LaTeX expression LHS")
    expression_latex_rhs = StringField(label="LaTeX expression RHS")

which gets instantiated in to_add_expression and then used in
https://github.com/allofphysicsgraph/ui_v8_website_flask_neo4j/blob/gh-pages/webserver/templates/property-graph/expression_create.html in

{{ form.expression_latex_lhs(id="input_latex_lhs") }} = 
{{ form.expression_latex_rhs(id="input_latex_rhs") }}

In the HTML I'd like to collect the relation, e.g.,

{{ form.expression_latex_lhs(id="input_latex_lhs") }} 
{{ form.expression_latex_lhs(id="input_latex_relation") }} 
{{ form.expression_latex_rhs(id="input_latex_rhs") }}

which means in the class I'd like to add

expression_relation = SelectField("relation", choices=self.list_of_relation_dropdown_tuples)

However, I wasn't able to figure out how to dynamically specify the list of tuples when instantiating the class.