Python default value if none

Code example for setting default values in Python when there is none

One possible solution is to set the default value for a field. For instance, the default for a particular field can be set to a specific value, and that field will be assigned that value if no other value is specified. Alternatively, an optional field can be included with the default value, or a required field can be specified with a default value. In the latter case, the default value can be assigned from a callable function.

How to print the default value if argument is None in python

def f(name): print(name or 'Hello Guest') def A(name=None): f(name) A() Out: "Hello Guest" A("Hello World") Out: "Hello World" 

To avoid repetition of the name variable in the function, it can be reassigned at the start of the function. This will ensure that the variable is used consistently throughout the function. name = name or «Hello Guest»

The optimal approach would be to employ a common default setting.

DEFAULT_NAME = "Hello Guest" def f(name=DEFAULT_NAME): print(name) def A(name=DEFAULT_NAME): f(name) 

One option to consider is to utilize inspect.signature for storing default value.

def f(name='Hello Guest'): print(name or inspect.signature(f).parameters['name'].default) def A(name=None): f(name) A() # Hello Guest 

To simplify the process (making it shorter and without the use of additional libraries), albeit with some potential limitations.

def f(name='Hello Guest'): print(name or f.__default__[0]) def A(name=None): f(name) A() # Hello Guest 

Python: Assign Value if None Exists, 8 Answers. Sorted by: 138. You should initialize variables to None and then check it: var1 = None if var1 is None: var1 = 4. Which can be written in one line as: var1 = 4 if var1 is None else var1. or using shortcut (but checking against None is recommended) var1 = var1 or 4. Usage examplevar1 = 4 if var1 is None else var1Feedback

Pydantic: How to pass the default value to a variable if None was passed?

In order to configure the model, it is necessary to activate the validate_assignment option.

from typing import Optional from pydantic import BaseModel, validator class User(BaseModel): name: Optional[str] = '' password: Optional[str] = '' class Config: validate_assignment = True @validator('name') def set_name(cls, name): return name or 'foo' user = User(name=None, password='some_password', ) print("Name is ", user.name) user.name = None print("Name is ", user.name) 

As the question was asked perfectly, I felt compelled to provide a more extensive example. After all, there are numerous approaches to dynamically assigning a value.

Although Alex’s answer is accurate, it is only applicable when the Field inherits a dataclass directly. In other words, this method will not function properly in cases such as the one presented.

class User(BaseModel): name: Optional[str] = "" password: Optional[str] = "" class Config: validate_assignment = True @validator("name") def set_name(cls, name): return name or "bar" user_dict = user_one = User(**user_dict) Out: name='' password='so_secret' 

Validate Always

Validators are not invoked for fields when there is no input provided for performance reasons. However, in cases where it is necessary to establish a dynamic default value, setting it to «True» is appropriate.

class User(BaseModel): name: Optional[str] = "" @validator("name", pre=True, always=True) def set_name(cls, name): return name or "bar" In: user_one = User(name=None) In: user_two = User() Out: name='bar' Out: name='bar' 

Although always=True is useful, it should be noted that pydantic’s validation of the default None could produce an error.

By setting the field to True , it will be called before any validation errors occur. The default value for a validator pre is False , in which case they are called after the field validation.

Using Config

But this has some disadvantages.

class User(BaseModel): name: Optional[str] = "" class Config: validate_assignment = True @validator("name") def set_name(cls, name): return name or "foo" In: user = User(name=None) Out: name='foo' 

Setting the value to None results in correct dynamic value retrieval, but it may fail in certain scenarios, such as when it is fully empty.

Once more, it is required to configure in order for it to function properly.

Using default_factory

If you need to establish a default value, such as UUID or datetime, default_factory could be very beneficial. However, it is important to note that Callable arguments cannot be assigned to the default_factory in such cases.

class User(BaseModel): created_at: datetime = Field(default_factory=datetime.now) In: user = User() Out: created_at=datetime.datetime(2020, 8, 29, 2, 40, 12, 780986) 

Many ways to assign a default value

One approach is to have a mandatory field, id , that comes with a preset value.

class User(BaseModel): id: str = uuid.uuid4() 

The second approach involves including a field, which is not mandatory, but may be used if needed. This field has a default value and is referred to as id .

class User(BaseModel): id: Optional[str] = uuid.uuid4() 

To implement the third method, it is necessary to have a field labeled id that is mandatory and has a predetermined default value.

class User(BaseModel): id: str = Field(default=uuid.uuid4()) 

One way to generate on-demand values such as unique UUIDs or Timestamps is by using a required field with a default value from a callable, which is described in Method #4. For more details, refer to @yagiz-degirmenci’s response.

class User(BaseModel): id: str = Field(default_factory=uuid.uuid4) # uuid.uuid4 is not executed immediately 

Python — How to pass the default value to a variable if, Method #3: A required id field with default value. class User(BaseModel): id: str = Field(default=uuid.uuid4()) Method #4: A required id field with default value from callable. This is useful for generating on-demand values such as unique UUIDs or Timestamps. See @yagiz-degirmenci answer.

How to set all default parameter values to None at once

To establish a simple function, the following code can be assigned: __defaults__ .

def foo(a, b, c, d): print (a, b, c, d) # foo.__code__.co_varnames is ('a', 'b', 'c', 'd') foo.__defaults__ = tuple(None for name in foo.__code__.co_varnames) foo(b=4, d=3) # prints (None, 4, None, 3) 

To make None the default for every argument, a decorator approach is necessary. If the concern is only with Python 3, inspect.signature is a viable solution.

def function_arguments_default_to_None(func): # Get the current function signature sig = inspect.signature(func) # Create a list of the parameters with an default of None but otherwise # identical to the original parameters newparams = [param.replace(default=None) for param in sig.parameters.values()] # Create a new signature based on the parameters with "None" default. newsig = sig.replace(parameters=newparams) def inner(*args, **kwargs): # Bind the passed in arguments (positional and named) to the changed # signature and pass them into the function. arguments = newsig.bind(*args, **kwargs) arguments.apply_defaults() return func(**arguments.arguments) return inner @function_arguments_default_to_None def x(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z): print(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z) x() # None None None None None None None None None None None None None None # None None None None None None None None None None None None x(2) # 2 None None None None None None None None None None None None None # None None None None None None None None None None None None x(q=3) # None None None None None None None None None None None None None None # None None 3 None None None None None None None None None 

By modifying the signature manually, you will sacrifice the ability to introspect the function.

I have a feeling that there might be more effective solutions to resolve the issue or even prevent it altogether.

Is there shorthand for returning a default value if None in, 4 Answers. Note that this also returns «default» if x is any falsy value, including an empty list, 0, empty string, or even datetime.time (0) (midnight). This will return «default» if x is any falsy value, e.g. None, [], «», etc. but is often good enough and cleaner looking. x or default is an antipattern!

Function default value not defined

It is possible to configure the preset value as None .

def my_function(a=10, b=None): if b is None: b = a 

The default setting for a is 10 , while b is automatically assigned a if not manually changed.

To include None along with another default, choose a distinctive one to serve as a sentinel. A commonly used convention is to use an example of object() .

_sentinel = object() def my_function(a=10, b=_sentinel): if b is _sentinel: b = a 

By calling my_function(11, None) , b will be assigned None . It is not necessary to specify b when calling it, for instance, my_function() or my_function(42) . In case b is called without specifying a , it will be assigned the same value as before.

Parameters without a default value, except for keyword parameters, must be specified.

The function «my_function(a,b)» requires two positional arguments to be passed as it does not have any default values assigned. Therefore, it cannot be called without providing the required arguments.

One of the primary concerns is how to assign the value of the first argument to the second argument if it is not specified.

There are two way for this:

def my_function(a=10, **kwargs): b = kwargs.get('b', a) 

sentinel as default Value

_sentinel = object() def my_function(a=10, b=_sentinel): if b is _sentinel: b = a 

It seems that defining my_function in such a manner is not possible. However, utilizing a decorator can conceal the calculation of default values.

import functools def my_decorator(f): @functools.wraps(f) def wrapper(a=10, b=None): if b is None: b = a return f(a, b) return wrapper 

You can define your function in the following manner.

@my_decorator def my_function(a, b): return (a, b) 

Your function can be utilized with either no parameters, a single parameter, or two parameters.

>>> print(my_function()) (10, 10) >>> print(my_function(5)) (5, 5) >>> print(my_function(5, 12)) (5, 12) 

How to apply default value to Python dataclass field, I want to setup a simple dataclass with a some default values like so: @dataclass class Specs1: a: str b: str = ‘Bravo’ c: str = ‘Charlie’ I would like to be able to get the default value for the second field but still set a value for the third one. I cannot do this with None because it is happily accepted as a value for my …

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Python default value if none

Last updated: Feb 18, 2023
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# Table of Contents

# Return a default value if None in Python

Use a conditional expression to return a default value if None in Python.

The conditional expression will return the default value if the variable stores None , otherwise the variable is returned.

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# 👇️ return a default value if the variable is None def example(): my_var = None return "default value" if my_var is None else my_var print(example()) # 👉️ "default value" # --------------------------------------------- # 👇️ return a default value if the variable is None my_var = None my_var = "default value" if my_var is None else my_var print(my_var) # 👉️ default value 

Conditional expressions are very similar to an if/else statement.

In the examples, we check if the variable stores None, and if it does, we return the string default value , otherwise, the variable is returned.

You can use this approach to return a default value if None from a function, or to reassign a variable to a default value if it currently stores None .

Alternatively, you can switch the order of conditions.

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# 👇️ return a default value if the variable is None def example(): my_var = None return my_var if my_var is not None else 'default value' print(example()) # 👉️ "default value" # --------------------------------------------- # 👇️ return a default value if the variable is None my_var = None my_var = my_var if my_var is not None else 'default value' print(my_var) # 👉️ default value 

The code sample first checks if the variable is not None , in which case it returns it.

Otherwise, a default value is returned.

# Return a default value if None using the boolean OR operator

An alternative approach is to use the boolean OR operator.

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# 👇️ return a default value if the variable is None def example(): my_var = None return my_var or "default value" print(example()) # 👉️ "default value" # --------------------------------------------- # 👇️ return a default value if the variable is None my_var = None my_var = my_var or "default value" print(my_var) # 👉️ "default value" 

The expression x or y returns the value to the left if it’s truthy, otherwise, the value to the right is returned.

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# 👇️ default value print(None or 'default value') # 👇️ hello print('hello' or 'default value') 

However, this approach does not explicitly check for None .

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