check if query exists using peewee

You can use .exists():

query = User.select().where(User.username == 'charlie')
if query.exists():
    # A user named "charlie" exists.
    cool()

http://docs.peewee-orm.com/en/latest/peewee/api.html?highlight=exists#SelectBase.exists

Alternatively, if you want to check if e.g. some other table refers this record, you can use WHERE EXISTS (subquery) clause. It is not supported natively by PeeWee, but it can be easily constructed:

subquery = Child.select(Param('1')).where(Child.parent == Parent.id)
parents_with_children = Parent.select().where(
    Clause(SQL('EXISTS'), subquery))

It is equivalent to the following SQL:

SELECT * FROM parent
WHERE EXISTS (SELECT 1 FROM child
              WHERE child.parent_id = parent.id);

Here I used SELECT 1 for subquery to avoid fetching unneeded information (like child.id). Not sure if such optimization is actually required.

UPD (Feb 2022)

After more than 5 years of peewee evolution, it looks like the Clause class is gone.

The following code may work (I didn't have a chance to test it though):

subquery = Child.select(Param('1')).where(Child.parent == Parent.id)
parents_with_children = Parent.select().where(
    NodeList((SQL('EXISTS'), subquery)))

If you just need to check existence use the accepted answer.

If you are going to use the record if it exists you can make use of Model.get_or_none() as this removes the need to use a try/catch and will not create a record if the record doesn't exist.

class User(peewee.Model):
    username = peewee.CharField(unique=True)

user = User.get_or_none(username='charlie')
if user is not None:
    # found user, do something with it
    pass