Handling null and undefined values
How to use the null coalescing operator to handle null and undefined values.
The null coalescing operator is a binary operator that returns its left-hand value if it's not null; otherwise, it returns its right-hand value. You can think of it as a way to provide a default value when the left-hand value is null.
"{{ null ?? now() | date('yyyy-MM-dd') }}"
In this example, since the left-hand side of the ?? operator is null, the right-hand side will be returned. The now() function will be called, and the result will be formatted as a date string in the yyyy-MM-dd format.
Processing date values
Imagine that you have a flow that processes data between two dates. You want to provide default values for the start and end dates if they're not provided as inputs. You can use the null coalescing operator to set default values for the start and end dates. In this example, the start date is set to one month ago, and the end date is set to today, effectively processing data for the last month by default.
id: process_data_between_dates
namespace: company.team
inputs:
- id: start_date
type: DATE
required: false
description: Start date to fetch data from
- id: end_date
type: DATE
required: false
description: End date to fetch data from
variables:
start_date: "{{ inputs.start_date ?? now() | dateAdd(-1, 'MONTHS') | date('yyyy-MM-dd') }}"
end_date: "{{ inputs.end_date ?? now() | date('yyyy-MM-dd') }}"
tasks:
- id: process_data_between_dates
type: io.kestra.plugin.core.log.Log
message: processing data from {{ render(vars.start_date) }} to {{ render(vars.end_date) }}
Note that you need to use the render function to recursively render variables containing Pebble expressions.
Providing default values for optional and undefined inputs
The null-coalescing operator ?? will return the right-hand value if the left-hand value is null or undefined (e.g. an input or a variable that have not been defined). This behavior is useful when you want to provide default values for optional inputs and for dynamic properties that may not be defined.
If you want to return the right-hand side only if the left-hand side is undefined, you can use the ??? operator instead of ??.
The example below shows how to use both ?? and ??? operators to set defaults for optional or undefined values.
id: provide_default_values
namespace: company.team
inputs:
- id: optional_input
type: STRING
required: false
description: An optional input
tasks:
- id: coalesce_optional_input
type: io.kestra.plugin.core.log.Log
message: |
Expression: inputs.optional_input ?? 'mydefault'
Left-hand value: null
Right-hand value: 'mydefault'
Operator used: '??'
This expression "{{ inputs.optional_input ?? 'mydefault' }}" will return 'mydefault'
because the coalesce-operator '??' returns the right-hand value
if the left-hand value is null or undefined.
Only if you provide a value at runtime, that value will be used instead of 'mydefault'.
- id: coalesce_undefined_input
type: io.kestra.plugin.core.log.Log
message: |
Expression: inputs.undefined_input ?? 'mydefault'
Left-hand value: undefined
Right-hand value: 'mydefault'
Operator used: '??'
The expression "{{ inputs.undefined_input ?? 'mydefault' }}" will return 'mydefault'
because the coalesce-operator '??' returns the right-hand value
if the left-hand value is null or undefined.
- id: coalesce_only_undefined_input_1
type: io.kestra.plugin.core.log.Log
message: |
Expression: inputs.undefined_input ??? 'mydefault'
Left-hand value: undefined
Right-hand value: 'mydefault'
Operator used: '???'
The expression "{{ inputs.undefined_input ??? 'mydefault' }}" will return 'mydefault'
because he undefined-coalesce-operator '???' returns the right-hand value
if the left-hand value is undefined.
- id: coalesce_only_undefined_input_2
type: io.kestra.plugin.core.log.Log
message: |
Expression: inputs.optional_input ??? 'mydefault'
Left-hand value: null
Right-hand value: 'mydefault'
Operator used: '???'
The expression "{{ inputs.optional_input ??? 'mydefault' }}" will return "" i.e. no value aka null value
because optional_input is defined and the undefined-coalesce-operator '???' only returns the right-hand value
if the left-hand value is undefined.
- id: both_operators_combined
type: io.kestra.plugin.core.log.Log
message: |
Expression: (inputs.optional_input ??? 'mydefault') ?? 'other_default'
Left-hand value: null
Right-hand value: 'other_default'
Operator used: '??'
The expression "{{ (inputs.optional_input ??? 'mydefault') ?? 'other_default' }}" will return 'other_default'
because the first expression using the undefined-coalesce-operator '???' will return null,
and the coalesce-operator '??' will return the default value 'other_default'.
Processing Trigger values
When using a Trigger, you can use the {{ trigger }} expression inside of your flow. However, this expression is undefined if you execute your flow manually.
Here's an example of a Webhook trigger that might receive a body of data. You can use the null coalescing operator to handle when the body of data is different to what's expected:
id: webhook_example
namespace: company.team
tasks:
- id: hello
type: io.kestra.plugin.core.log.Log
message: "{{ trigger.body.dataField ?? 'data' }}"
triggers:
- id: webhook
type: io.kestra.plugin.core.trigger.Webhook
key: abcdefg
You can also use this for Schedule triggers. Here's an example that uses the date when the Schedule trigger starts an execution but in combination with the null coalescing operator to use an input value if an execution is started manually.
id: scheduling
namespace: company.team
inputs:
- id: country
type: STRING
defaults: US
- id: date
type: DATETIME
required: false
defaults: 2023-12-24T14:00:00.000Z
tasks:
- id: check_if_business_date
type: io.kestra.plugin.scripts.python.Commands
warningOnStdErr: false
namespaceFiles:
enabled: true
commands:
- python schedule.py "{{ trigger.date ?? inputs.date }}" {{ inputs.country }}
beforeCommands:
- pip install workalendar
taskRunner:
type: io.kestra.plugin.core.runner.Process
- id: log
type: io.kestra.plugin.core.log.Log
message: business day - continuing the flow...
triggers:
- id: schedule
type: io.kestra.plugin.core.trigger.Schedule
cron: 0 14 25 12 *
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