Python argument default self

How to set default argument with self in python def?

This is solvable in our case by avoiding the default argument or mitigated by default argument resolution like this: P.S. Keep in mind that the default argument, is shared by the different invocations therefore usage of default mutable parameters (and in Python functions are first-class objects and mutable) should be with care. Using a function as a default argument would work, but the general Best-Practice still holds: When using a concrete value for the argument in the method signature: It creates dependency between the function logic and the default argument selection, which is exactly what you want to avoid.

How to set default argument with self in python def?

I am trying to call the __init__ variable as a default variable but it is not calling

class test1: def __init__(self,name): self.name = name def method1(self,name2=self.name): return name2 

If I call the function like

from test import test1 ma = test1("mkHun") va = ma.method1() # Here I am expected output is mkhun ca = ma.method1("new name") 

In JavaScript this features is called as default parameters. Is this features is available in python or something problem with my code?

Here’s one way to achieve similar functionality. Others may have a more pythonic way. Code inspired by the docs here

class test1: def __init__(self, name): self.name = name def method1(self, newName=None): if newName is not None: self.name = newName return self.name ma = test1("mkHun") va = ma.method1() # Here I am expected output is mkhun print(va) ca = ma.method1("new name") print(ca) 

class test1: def __init__(self,name): self.name = name def method1(self, name2="This would work"): """You can already use self.name here without taking it from the method argument since self.name always exists, you don't need to put it in the method arguments""" return name2 ma = test1("mkHun") va = ma.method1() # Here I am expected output is mkhun ca = ma.method1("new name") print(ma.name, "--->", va,"--->", ca) 

self.variable is in reach for every method you define inside the class but @staticmethod, so you can simply give it a None as default argument and inside your method if the argument is None use the self.name and if it is not None and the argument indeed exist, use it instead. Hope it helps.

Python — How to distinguish default argument and, The common practice in such cases is to use specific sentinel value you never want to return. class Sentinel(): pass _sentinel = Sentinel()

Defining Python Functions With Default and Optional Arguments

Defining your own functions is an essential skill for writing clean and effective code. In this video
Duration: 11:03

How can I set a default for star arguments?

I have a function that accepts *args , but I would like to set a default tuple, in case none are provided. (This is not possible through def f(*args=(1, 3, 5)) , which raises a SyntaxError .) What would be the best way to accomplish this? The intended functionality is shown below.

f() # I received 1, 2, 3! f(1) # I received 1! f(9, 3, 72) # I received 9, 3, 72! 

The following function g will provide the correct functionality, but I would prefer *args .

def g(args=(1, 2, 3)): return "I received <>!".format(', '.join(str(arg) for arg in args)) g() # I received 1, 2, 3! g((1,)) # I received 1! g((9, 3, 72)) # I received 9, 3, 72! 

You could check whether args are truthy in your function:

def g(*args): if not args: args = (1, 2, 3) return "I received <>!".format(', '.join(str(arg) for arg in args)) 

If no args are passed to the function, it will result in a empty tuple, which evaluates to False .

If no arguments are received, args will be an empty tuple. You can’t add a default value in the method signature itself, but you can check if args is empty and replace it with a fallback value inside the function.

def g(*args): if not args: args = (1, 2, 3) return 'I received <>!'.format(', '.join(str(arg) for arg in args)) 

def g(*args): if not args: args = [1, 3, 5] return "I received <>!".format(', '.join(str(arg) for arg in args)) 

How do I add default parameters to functions when using type hinting?, Edit: I didn’t know how default arguments worked in Python, but for the sake of this question, I will keep the examples above.

In Python, is it bad form to set an argument’s default value to be a function?

def wheels_factory_1(): . def wheels_factory_2(): . def make_car(wheels_factory_fn=wheels_factory_1): car = Car() car.add_wheels(wheels_factory_fn()) return car car1 = make_car() car2 = make_car(wheels_factory_fn=wheels_factory_2) 

Is it a common practice to set the default of wheels_factory_fn to be a function, like wheels_factory_1 ? I haven’t seen this in a code base before, just wondering if there’s some potential issue with doing this.

The reason why I want to structure code like this is to make it easy to do dependency injection.

Any default value which does not change over time should be good.

On the other hand, consider this example:

when called with an argument, behaves like a pure function :

>>> foo([1,2,3]) [1, 2, 3, 1] >>> foo([1,2,3]) [1, 2, 3, 1] 

but when called with the default value, it is something else.

which may or may not be intended.

By using a function as the default value, you should be ok, as those are rarely modified in such a way.

Setting the default argument value to functions is common practice in machine learning libraries. Some image processing functions or transformations are set in the arguments which you have to explicitly mention.

It would be good if you can insert the default function value while instantiating the class.

class Car: def __init__(self, function = default_func): pass 

This would be easy for injecting the dependencies. reference: https://medium.com/@shivama205/dependency-injection-python-cb2b5f336dce

For dependency injection, you can use dependency-injector .

Using a function as a default argument would work, but the general Best-Practice still holds:

When using a concrete value for the argument in the method signature:

1. It creates dependency between the function logic and the default argument selection, which is exactly what you want to avoid.

This is solvable in our case by using the dependency injection, instead of the default argument.

1. In case the function with the default argument is moved to another class, and one day you want to inherit from that class and change the default argument for the inheriting method, you shall set a default argument at the inherited class method too (problematic). The same applies when you have another function that calls the function with the default argument, and you want to change the default argument in it.

This is solvable in our case by avoiding the default argument or mitigated by default argument resolution like this:

def method(x=None): x = value if value else DEFAULT_VALUE 

Keep in mind that the default argument, is shared by the different invocations therefore usage of default mutable parameters (and in Python functions are first-class objects and mutable) should be with care.

In Python, is it bad form to set an argument’s default value to be a, This is solvable in our case by using the dependency injection, instead of the default argument. In case the function with the default argument

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Python Default Arguments

This tutorial introduces the concept of default arguments in Python.

A default argument is a function argument that takes on a default value if you don’t pass an explicit value for when calling the function. For example, the function definition def f(x=0): allows you to call it with or without the optional argument x —valid calls are f(2) , f(4) , or even f() . But if you don’t pass the optional argument, it’ll just assign the default value 0 to argument x .

Examples Default Argument

def f(x=0): print(x) f(10) # 10 f(-2) # -2 f('hello world') # hello world ######################## # DEFAULT ARGUMENT # ######################## f() # 0

Application: When to Use Default Arguments?

Suppose, you have created a Python command line tool for your business. The tool requires user confirmation for different activities like writing or deleting files.

To avoid redundant code, you have implemented a generic function that handles the interaction with the user. The default behavior should consist of three steps.

• (1) You ask (prompt) the user a question.
• (2) The user puts in some response.
• (3) As long as the response is invalid, the function repeats up to four times–each time printing a reminder ‘Try again: yes or no?’ .

The number of repetitions and the reminder should be customizable via the parameters.

To achieve this, you can specify default arguments as given in the following code snippet. You can use the default parameters by calling ask_ok(‘May we send you a free gift?’) . Or you can overwrite them in the order of their definition (one, several, or all parameters).

def ask_ok(prompt, retries=4, reminder='Try again: yes or no?'): while retries>0: ok = input(prompt) if ok in ('y', 'yes'): return True if ok in ('n', 'no'): return False retries = retries - 1 print(reminder) ask_ok('May we send you a free gift?')

Let’s check how you understand this concept of default arguments.

Puzzle Default Arguments

Is ask_ok(‘Howdy?’, 5) a valid function call?

It is interesting that only 50% of all finxter users solve this puzzle: they seem to guess the answer. Partial replacement of default arguments is a new feature to most of the users. Is it new to you?

Mastering these basic language features will lift you to the level of an advanced coder.

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