calling append() on a ListField changes the default value

[lingthio]

Thanks for this great package!

On object creation, where:

• an attribute's Field.default has been defined, and
• an attribute value is not specified in **kwargs

the object is created with this attribute value set to the Field.default.

For ListField attributes, this causes a problem, because list operations such as
append() will modify the Field.default and subsequent object creations will
behave unexpectedly.

I have included a code illustration below.

The obvious fix is to always initialize an object with a COPY of the Field.default.

from flask import Flask
from flask_mongoalchemy import MongoAlchemy

# Setup Flask
app = Flask(__name__)

# Setup MongoAlchemy
app.config.update(MONGOALCHEMY_DATABASE='issue_db')
db = MongoAlchemy(app)

# Define Item Document
class Item(db.Document):
    name = db.StringField(default='')
    colors = db.ListField(db.StringField(), default=[])

# Clear Item collection
db.session.clear_collection(Item)

# Create Documents
item1 = Item(name='Item One')
db.session.add(item1)
# item1.colors.append() modifies the Field default
item1.colors.append('red')
item1.colors.append('blue')

item2 = Item(name='Item Two')
db.session.add(item2)
db.session.flush()

# item2 is created with the modified Field default
assert item2.colors==[], 'item2.colors='+str(item2.colors)+' instead of []'

If users are looking for a temporary workaround:

item1.colors = item1.colors + ['red']  # this ensures a copy is assigned
item1.colors = item1.colors + ['blue']  # this ensures a copy is assigned

[fsouza]

Hey @lingthio, thanks for reporting. Unfortunately, this is an issue in mongoalchemy itself. We can fix it on flask-mongoalchemy, but ideally this would be a fix upstream.