What's in Airflow's context dictionary?

Airflow’s context dictionary can be found in the get_template_context method, in Airflow’s models.py.

{
    'dag': task.dag,
    'ds': ds,
    'ds_nodash': ds_nodash,
    'ts': ts,
    'ts_nodash': ts_nodash,
    'yesterday_ds': yesterday_ds,
    'yesterday_ds_nodash': yesterday_ds_nodash,
    'tomorrow_ds': tomorrow_ds,
    'tomorrow_ds_nodash': tomorrow_ds_nodash,
    'END_DATE': ds,
    'end_date': ds,
    'dag_run': dag_run,
    'run_id': run_id,
    'execution_date': self.execution_date,
    'prev_execution_date': prev_execution_date,
    'next_execution_date': next_execution_date,
    'latest_date': ds,
    'macros': macros,
    'params': params,
    'tables': tables,
    'task': task,
    'task_instance': self,
    'ti': self,
    'task_instance_key_str': ti_key_str,
    'conf': configuration,
    'test_mode': self.test_mode,
    'var': {
        'value': VariableAccessor(),
        'json': VariableJsonAccessor()
    }
}

An explanation of each item is found in the documentation under Macros.

Incidentally, you can generate the context from a TaskInstance.

context = TaskInstance(
    task=task,
    execution_date=datetime.now()
).get_template_context()