Pythonic way set variables if none in init

Question

What's the most pythonic or elegant way of achieving this?

def __init__(self, connection=None, some=None, thing=None, else=None):
   if connection is None:
      self.connection = SetConnection()
   else:
      self.connection = connection
.
.

If I have multiple input args like above for which I would like to call another class to instantiate. Any good way to keep it clean looking without being verbose?

I keep find it amazing how it is always the "simple" questions that raise so much traffic and ideas — DeepSpace
– DeepSpace, Commented Dec 13, 2018 at 21:03
you can use isinstance to check if connection is indeed the class/type you need and not something else. — Pedro Lobito
– Pedro Lobito, Commented Dec 13, 2018 at 21:23

Jab · Accepted Answer · 2018-12-13 20:55:42Z

4

You could use a binary operator:

def __init__(self, connection=None, some=None, thing=None, else=None):
    self.connection = connection or SetConnection()

If connection is None (False) it will run SetConnection().

edited Dec 13, 2018 at 20:55

answered Dec 13, 2018 at 20:54

Jab

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9 Comments

DeepSpace Over a year ago

This will only work as long as False, 0, empty list etc are not possible values. A single-liner that solves this is self.connection = connection if connection is not None else SetConnection()

jasonharper Over a year ago

Note that this assumes that all valid values of the parameter are truthy.

Jab Over a year ago

True, this does make assumptions based on input. Though he was asking for "Any good way to keep it clean looking without being verbose"

Daniel Pryden Over a year ago

While this approach has its limitations (indeed, it won't work if connection can have a valid non-None value that is not truthy, or if the SetConnection class defines __bool__ in an unusual way), it's also in my experience very idiomatic. I've definitely seen a lot of code use or rather than a ternary expression for cases like this.

Jab Over a year ago

It makes sense when you take his question's confinements into consideration. Thanks for taking my side, or has it's weakneses but it does make for "clean looking" code and this is what he asked for

|

Daniel · Accepted Answer · 2018-12-13 20:57:53Z

1

Use ternary operator

def __init__(self, connection=None, some=None, thing=None, else=None):
   self.connection = SetConnection() if connection is None else connection

answered Dec 13, 2018 at 20:57

Daniel

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1 Comment

Joel Over a year ago

Although other answer shows "simpler" solution, I ended up using this as it will not mislead when inputs are 0, False, [], (), {} and ''

jez · Accepted Answer · 2018-12-13 20:58:45Z

1

If you don't need to record, for any further reason, the fact that connection was originally passed as None, then you could overwrite that name:

if connection is None: connection = SetConnection()
self.connection = connection

answered Dec 13, 2018 at 20:58

jez

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1 Comment

TemporalWolf Over a year ago

While this is a valid syntax, please don't format your code like this. Put that inner line on it's own, indented line. Compound statements are generally discouraged

Pedro Lobito · Accepted Answer · 2018-12-13 21:54:10Z

0

My 2c using isinstance :

def __init__(self, connection=None, some=None, thing=None, else=None):
   if isinstance(connection, "class/type of connection") :
       self.connection = connection    
   else:
       self.connection = SetConnection()

Old answer:

self.connection = connection if connection else SetConnection()

PS: I'm not aware of the class or type of connection, if you also don't know it, usetype(connection)

edited Dec 13, 2018 at 21:54

answered Dec 13, 2018 at 21:10

Pedro Lobito

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2 Comments

jez Over a year ago

Same debate applies as to Jaba's answer: this is probably OK given the context, but it makes the unstated assumption that 0, False, [], (), {} and '' are not among the valid possibilities for connection.

Pedro Lobito Over a year ago

Noted. One solution would be using isinstance to assure connection is the instance we expect it to be.

keepAlive · Accepted Answer · 2018-12-15 13:40:21Z

0

Just for fun, why not do

def __init__(self, **kwargs):
    self.connection = kwargs.get('connection', SetConnection())

Which puts aside problems related to falsy values while being pythonic. That being said, it has other drawbacks for sure, e.g. those related to the idea of auto-documentability of the code.

answered Dec 15, 2018 at 13:40

keepAlive

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