Can't inspect code from an object in Python

Question

Here is the beginning of my code:

import os
from flask import Flask, render_template, session, redirect, url_for, flash
from flask_bootstrap import Bootstrap
from flask_moment import Moment
from flask_wtf import FlaskForm
from wtforms import StringField, SubmitField
from wtforms.validators import DataRequired
from flask_sqlalchemy import SQLAlchemy

basedir = os.path.abspath(os.path.dirname(__file__))

app = Flask(__name__)
app.config['SECRET_KEY'] = 'hard to guess string'
app.config['SQLALCHEMY_DATABASE_URI'] =\
    'sqlite:///' + os.path.join(basedir, 'data.sqlite')
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

bootstrap = Bootstrap(app)
moment = Moment(app)
db = SQLAlchemy(app)

a=db.Model

class Role(db.Model):
    __tablename__ = 'roles'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), unique=True)
    users = db.relationship('User', backref='role')

    def __repr__(self):
        return '' % self.name

As i do not see an init method in Role, i suppose it is in parent class db.Model. So i wanted to see how this class is defined. I created the object named 'a' in the upper script and in a python terminal run:

In [11]: inspect.getmodule(a)
Out[11]: 

In [12]: inspect.getsource(a)
...
C:\Python35\lib\inspect.py in findsource(object)
    785             return lines, candidates[0][1]
    786         else:
--> 787             raise OSError('could not find class definition')
    788
    789     if ismethod(object):

OSError: could not find class definition

Why do i get this error ? The class seemed to come from a python script (not C code). Moreover in bonus if someone could provide me the __init__method of the parent class it would be wonderful!

Thanks (i am autodicact in code so any advices to inspect/debug code would be interesting, perhaps an other dedicated question...).

user_8773585 · Answer

So I'm not entirely sure what's going on - the exacts of the inspect module are pretty complex.

However I think one thing that might be causing issues is your querying an instance of a class: a and not the actual class.

If you do query the class you get:

>>> t = type(a)
>>> t
flask_sqlalchemy.model.DefaultMeta

>>> inspect.getsourcefile(t)
'...\site-packages\flask_sqlalchemy\model.py'

>>>  inspect.getsource(t)
'class DefaultMeta(NameMetaMixin, BindMetaMixin, DeclarativeMeta):
    pass
'

So it seems to make sense that inspecting the class returns the appropriate source file. What I don't understand is why does inspecting an instance of the class return `'...\site-packages\sqlalchemy\ext\declarative\api.py'.

If it doesn't return the file containing the class definition, I would expect it to return the file where it's instantiated, but that would be the script that you run not this api.py file.

Can't inspect code from an object in Python

1 Answers