Flask导入错误

Question

I have workspace and task blueprint in flask with workspace view workspace/apis.py and task view task/apis.py

I want to import Task class from Task model in workspace/apis.py but i can't.

workspace model:

class Workspace(db.Model):
    __tablename__ = 'workspace'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.Unicode(50), nullable=False)
    created_on = db.Column(db.DateTime, default=datetime.utcnow)
    users = db.relationship('User', secondary=workspace_users,
                        backref=db.backref('workspaces', lazy='dynamic'),        lazy='dynamic')
    owners = db.relationship('User', secondary=workspace_owners,
                         backref=db.backref('own_workspaces', lazy='dynamic'), lazy='dynamic')
    teams = db.relationship('Team', cascade="all,delete", backref='workspace', lazy='dynamic')
    projects = db.relationship('Project', cascade="all,delete", backref='workspace', lazy='dynamic')
    tags = db.relationship('Tag', cascade="all,delete", backref='workspace', lazy='dynamic')
    user_costs = db.relationship('UserRate', backref='workspace', lazy='dynamic')
    clients = db.relationship('Client', backref='workspace', lazy='dynamic')

and task model:

class Task(db.Model):
    __tablename__ = 'task'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.Unicode(120), nullable=False)
    start_time = db.Column(db.DateTime, default=datetime.datetime.utcnow)
    end_time = db.Column(db.DateTime)
    second = db.Column(db.Integer)
    rate = db.Column(db.Float)
    status = db.Column(db.Boolean, default=True)
    project_id = db.Column(db.Integer, db.ForeignKey('project.id'))
    owner_id = db.Column(db.Integer, db.ForeignKey('user.id'))
    tags = db.relationship('Tag', secondary=task_tag,
                           backref=db.backref('tasks', lazy='dynamic'), lazy='dynamic')

in task view i can import Workspace class but on workspace view i can't import Task class.

error:

Traceback (most recent call last):
  File "./run.py", line 8, in <module>
    from project.application import create_app, db
  File "/home/itmard/git/toggle-dev/project/__init__.py", line 1, in <module>
    from project.application import create_app
  File "/home/itmard/git/toggle-dev/project/application.py", line 5, in <module>
    from project.apps.task.models import calculate_user_rate
  File "/home/itmard/git/toggle-dev/project/apps/task/__init__.py", line 12, in <module>
    from . import apis
  File "/home/itmard/git/toggle-dev/project/apps/task/apis.py", line 12, in <module>
    from project.apps.task.models import Task
  File "/home/itmard/git/toggle-dev/project/apps/task/models.py", line 7, in <module>
    from project.apps.user_rate.models import UserRate
  File "/home/itmard/git/toggle-dev/project/apps/user_rate/__init__.py", line 8, in <module>
    from . import views
  File "/home/itmard/git/toggle-dev/project/apps/user_rate/views.py", line 10, in <module>
    from project.apps.user_rate.forms import AddUserRateForm
  File "/home/itmard/git/toggle-dev/project/apps/user_rate/forms.py", line 11, in <module>
    from project.apps.workspace.models import workspaces
  File "/home/itmard/git/toggle-dev/project/apps/workspace/__init__.py", line 12, in <module>
    from . import apis
  File "/home/itmard/git/toggle-dev/project/apps/workspace/apis.py", line 12, in <module>
    from project.apps.task.models import Task
ImportError: cannot import name Task

project structure :

├── project
│   ├── application.py
│   ├── apps
│   │   ├── task
│   │   │   ├── apis.py
│   │   │   ├── forms.py
│   │   │   ├── __init__.py
│   │   │   ├── __init__.py~
│   │   │   ├── models.py
│   │   │   └── views.py
│   │   └── workspace
│   │       ├── apis.py
│   │       ├── forms.py
│   │       ├── __init__.py
│   │       ├── models.py
│   │       └── views.py

Answer 1

Think about the following scenario:

main.py

from a import whats_up
print whats_up()

Module a:

first = 1
from b import second
third = 3
def whats_up():
    return first + second + third

Module b:

second = 2
from a import first
print first
from a import third
print third

This code suffers the same problem.

Module a will, when being imported, have an object created for it. It will then have first assigned to that object. Next, Python will start importing module b . Note that we have not assigned third to module a yet!

When module b is loaded, it will assign second , then attempt to import first from a . This succeeds, and so 1 will be printed. Then it will attempt to import third from a . This is where the error occurs: a never finished its load to the point where it defined third .

Now let's look at your stack trace:

Your traceback looks like this:

Traceback (most recent call last):
  File "./run.py", line 8, in <module>
    from project.application import create_app, db
  File "/home/itmard/git/toggle-dev/project/__init__.py", line 1, in <module>
    from project.application import create_app
  File "/home/itmard/git/toggle-dev/project/application.py", line 5, in <module>
    from project.apps.task.models import calculate_user_rate
  File "/home/itmard/git/toggle-dev/project/apps/task/__init__.py", line 12, in <module>
    from . import apis
  File "/home/itmard/git/toggle-dev/project/apps/task/apis.py", line 12, in <module>
    from project.apps.task.models import Task
  ...
  File "/home/itmard/git/toggle-dev/project/apps/user_rate/forms.py", line 11, in <module>
    from project.apps.workspace.models import workspaces
  File "/home/itmard/git/toggle-dev/project/apps/workspace/__init__.py", line 12, in <module>
    from . import apis
  File "/home/itmard/git/toggle-dev/project/apps/workspace/apis.py", line 12, in    <module>
    from project.apps.task.models import Task

Note the section right before the ... : At this point, the tasks.models module is being imported. However, on line 7 of that, you then do this:

from project.apps.user_rate.models import UserRate

Before the Task model could fully be defined, you start loading the user_rate.models module, which itself has a bunch of definitions which, if you follow the stack trace, lead back to one of those modules also trying to import apps.tasks.models .

This is causing a circular dependency (eg, module 1 depends on module 2 which depends on module 3 which depends on module 1). You need to rethink how your modules depend on each other to solve this problem.