Python __repr__ and None
-
09-02-2021 - |
Question
I'm quite new to Python and currently I need to have a __repr__
for a SqlAlchemy class.
I have an integer column that can accept Null
value and SqlAlchemy converts it to None
.
For example:
class Stats(Base):
__tablename__ = "stats"
description = Column(String(2000))
mystat = Column(Integer, nullable=True)
What is the correct way to represent the "mystat" field in the __repr__
function when SqlAlchemy returns None
?
Solution
The __repr__
should return a string that describes the object. If possible, it should be a valid Python expression that evaluates to an equal object. This is true for built-in types like int
or str
:
>>> x = 'foo'
>>> eval(repr(x)) == x
True
If that's not possible, it should be a '<...>'
string uniquely describing the object. The default __repr__
is an example of this:
>>> class Foo(object):
pass
>>>
>>> repr(Foo())
'<__main__.Foo object at 0x02A74E50>'
It uses the object's address in memory to uniquely identify it. Of course address doesn't tell us much about the object so it's useful to override __repr__
and return a string describing the object's state.
The object's state is defined by other objects it contains so it makes sense to include their repr
in yours. This is exactly what list
or dict
do:
>>> repr(['bar', Foo()])
"['bar', <__main__.Foo object at 0x02A74710>]"
In your case, the state is in your Column
properties so you want to use their repr
. You can use the %r
formatting for this, it inserts a repr()
of the argument:
def __repr__(self):
return '<Stats: description=%r, mystat=%r>' % (self.description, self.mystat)
An equivalent using the new formatting:
def __repr__(self):
return '<Stats: description={0.description!r}, mystat={0.mystat!r}>'.format(self)
OTHER TIPS
I was trying to find a generic __repr__
method that could be used on any SQLAlchemy object and only found this page. So, I decided to write my own and here's what I've done:
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
if __debug__:
# monkey-patch in useful repr() for all objects, but only in dev
def tablerepr(self):
return "<{}({})>".format(
self.__class__.__name__,
', '.join(
["{}={}".format(k, repr(self.__dict__[k]))
for k in sorted(self.__dict__.keys())
if k[0] != '_']
)
)
Base.__repr__ = tablerepr
This extends the Base
class to print out the contents of the particular instance. So, now every object I create that extends the Base
will have a __repr__
method that prints more than just the class name and the instance hash.
EDIT: I added an if __debug__
as this change may cause a leak of information you may not want leaked in a production environment. I also added a sorted
so the display will be consistent.
Isn't the generic_repr decorator of sqlalchemy-utils providing a community based solution that suit your needs?
It leaves it as None.
maybe repr(mystat)
is what you want?
'Null' if mystat is None else mystat
The previous answer showing how to override Base.__repr__
was exactly what I needed. Thank you! Here it is re-written with f-strings for Python 3.7+ and overriding the flask-sqlalchemy db.Model
.
def override_default_repr(self):
class_name = self.__class__.__name__
attribs = ", ".join(
[
f"{k}={self.__dict__[k]!r}"
for k in sorted(self.__dict__.keys())
if k[0] != "_"
]
)
return f"<{class_name}({attribs})>"
db.Model.__repr__ = override_default_repr