Omitting the last comma in a series in a Jinja template

Jinja is a great Python template engine.This article explains how to omit the last comma in a series with Jinja.

The desired result is shown below—this is an argument list for an SQL stored procedure. Note that the Id column needs a zero default value and there cannot be a comma after the last column.

@Id int = 0,
@Name nvarchar(100),
@PrefixWithThe bit,
@Added datetime,
@Updated datetime

In the Libretto SQL Server Json schema there is a columnSqlDeclarations column meta token that produces this:

@Id int,
@Name nvarchar(100),
@PrefixWithThe bit,
@Added datetime,
@Updated datetime,

That’s close, but no cigar. It doesn’t assign the zero to the Id column and the trailing comma breaks things.

Assigning a zero to the Id is a special case, so (in my case) its worth a little manual work force the zero assignment (note also that the test for the id column is case insensitive—just in case):

{% for col in columns %}
{% if col.ColumnName.lower() == 'id' %}
@{{col.ColumnName}} {{col.DDLType}} = 0
{% else %}
@{{col.ColumnName}} {{col.DDLType}}
{% endif %}
{% endfor %}

That’s close, but the trailing comma remains.

Id int = 0,
@Name nvarchar(100),
@PrefixWithThe bit,
@Added datetime,
@Updated datetime,

To get the desired result, the template needs omit the trailing comma on the last element in the series. This template:

{% for col in columns %}
{% if col.ColumnName.lower() == 'id' %}
@{{col.ColumnName}} {{col.DDLType}} = 0{% if not loop.last %},{% endif %}
{% else %}
@{{col.ColumnName}} {{col.DDLType}}{% if not loop.last %},{% endif %}
{% endif %}
{% endfor %}

produces the correct results:

@Id int = 0,
@Name nvarchar(100),
@PrefixWithThe bit,
@Added datetime,
@Updated datetime

The Jinja templating engine has a built-in variable that tracks a loop index. The loop.last value conditions when the template emits the comma.

{% if not loop.last %},{% endif %}

This article is too long!

{% for column in columns -%}
    {% if column.PrimaryKey == 'True' %}
        {% if not loop.first %}, {% endif %}{{column.ColumnName}} = keyValue{{loop.index}}
    {% endif %}
{%- endfor %}

See the Jinja loop documentation for more info.

While it’s not directly a part of this issue, knowing how Jinja controls white space is helpful.

Also, don’t miss this Jinja tricks page in the Jinja docs.