10 Python Kwargs Best Practices

Python kwargs are a powerful tool that can help make your code more readable and concise. In this article, we’ll take a look at 10 Python kwargs best practices that can help you write better code.

Kwargs are often used in Python to specify optional arguments to a function. By using kwargs, you can make your code more flexible and easier to read.

1. Use kwargs to make your code more readable

Consider the following example:

def foo(**kwargs):
return kwargs[‘foo’]

This function is perfectly valid, but it’s not very readable. A better way to write this would be:

def foo(*, foo):
return foo

By using a keyword-only argument, we’ve made it clear that the foo argument is required, and we’ve given it a more descriptive name. This makes our code easier to understand, both for ourselves and for others.

Similarly, we can use kwargs to make our code more self-documenting. For example, consider the following code:

def foo(**kwargs):
# do something with kwargs

This code is perfectly valid, but it doesn’t tell us anything about what it does with kwargs. A more self-documenting way to write this would be:

def foo(**kwargs):
# process the keyword arguments

Now it’s clear that the function is doing something with the keyword arguments, without us having to look at the implementation.

2. Don’t use kwargs for required parameters

If you use kwargs for required parameters, then your function can be called without those parameters, and that can lead to unexpected behavior. For example, consider this code:

def my_func(a, b, c):
print(a, b, c)

my_func(1, 2)

This code will print out “1 2 None”, because the value of c is not provided. This can obviously lead to bugs, so it’s best to avoid using kwargs for required parameters.

3. Make sure you have a default value for every keyword argument in the function definition

If you don’t have a default value for a keyword argument, and the caller of the function doesn’t provide a value for that keyword argument, Python will raise a TypeError exception.

This is problematic because it can lead to unexpected behavior, and can cause your program to crash. Therefore, it’s important to always make sure you have a default value for every keyword argument in your function definition.

Not only does this help prevent unexpected behavior, but it also makes your code more readable and easier to understand.

4. Avoid mutable default arguments

When you use a mutable data type as a default argument, that data type is created only once when the function is first defined. This can lead to unexpected behavior if the data type is modified inside the function body without the caller being aware of it.

For example, consider the following code:

def func(arg1, arg2, lst=[]):
lst.append(arg1)
lst.append(arg2)
return lst

If we call this function like so:

result = func(1, 2)

We would expect the result to be [1, 2], but instead it’s [1, 2, 1, 2]. That’s because the default value for the lst argument is a list that is shared between all calls to the function, and each time we append to it, those changes are reflected in subsequent calls.

To avoid this issue, you should use a sentinel value as the default argument, like so:

def func(arg1, arg2, lst=None):
if lst is None:
lst = []
lst.append(arg1)
lst.append(arg2)
return lst

Now, each time we call the function, a new list will be created, and we won’t run into the same issue.

5. Be careful with variable scope when using kwargs

When you use kwargs in a function, the Python interpreter will create a new variable in the local namespace for each keyword argument. This is fine most of the time, but it can cause problems if you’re not careful.

For example, let’s say you have a function that takes two keyword arguments, and you want to use one of those arguments to control the behavior of the other. You might write something like this:

def my_function(arg1, arg2):
if arg1 == ‘foo’:
# do something with arg2
else:
# do something else with arg2

This looks fine at first glance, but it doesn’t work the way you might expect. The problem is that when the Python interpreter sees the if statement, it creates a new variable called arg1 in the local namespace. So, when the if statement tries to access the arg1 variable, it’s actually accessing the local arg1 variable, not the keyword argument.

To fix this, you need to be explicit about which namespace you want to access. The easiest way to do this is to use the globals() function:

def my_function(arg1, arg2):
if globals()[‘arg1’] == ‘foo’:
# do something with arg2
else:
# do something else with arg2

Now, the if statement will access the arg1 variable in the global namespace, which is what we want.

6. Use **kwargs if you need to handle named parameters that are not known beforehand

If you’re using **kwargs, then all of the named parameters that are passed to your function will be stored in a dictionary. This is important because it means you can access any parameter by name, and you don’t have to worry about the order in which they were passed.

Furthermore, if you need to handle default values for some of your parameters, then **kwargs is the way to go. All you need to do is check if the parameter is in the dictionary, and if not, then use the default value.

Overall, **kwargs is a very powerful tool that can make your code more flexible and easier to read. So, if you’re ever in a situation where you need to handle named parameters that are not known beforehand, then don’t hesitate to reach for **kwargs.

7. You can use *args and **kwargs together but *args must occur before **kwargs

The way Python handles function arguments is that it first assigns any positional arguments to the corresponding parameters in the function definition. Then, it assigns any keyword arguments to the parameters in the function definition.

If you were to try and use *args and **kwargs together but put **kwargs before *args, then Python would raise a TypeError because it wouldn’t know how to handle the keyword arguments.

Putting *args before **kwargs is considered best practice because it makes your code more explicit and easier to read.

8. The order of parameters does matter

When you use kwargs in a function call, the order of the parameters is important because it determines how the values are assigned to the keyword arguments. For example, consider the following code:

def my_func(a, b=1, c=2):
print(a, b, c)

my_func(1, c=3)

This code will output 1 3 2. Even though we explicitly set the value of c to 3, the value of b is still 1. This is because when Python sees that we have explicitly set the value of c, it will assign the value of 1 to b and the value of 3 to c.

If we had reversed the order of the parameters like this:

def my_func(a, c=2, b=1):
print(a, b, c)

my_func(1, c=3)

Then the code would output 1 1 3, because now Python knows to assign the value of 1 to b and the value of 3 to c.

So, when using kwargs, make sure to put the parameters in the correct order, so the values are assigned correctly.

9. Always include self as the first parameter of instance methods

When using kwargs, Python will automatically set the first parameter to the name of the instance (self). However, if you don’t include self as the first parameter, Python will raise a TypeError.

This is because Python doesn’t know what to set the first parameter to since it’s not explicitly defined. By always including self as the first parameter, you can avoid this error and ensure that your code will run smoothly.

10. Do not use kwargs for class attributes

When you use kwargs for class attributes, it can be difficult to debug because the order of the keyword arguments is not always the same. This can lead to errors that are hard to track down. In addition, using kwargs for class attributes can make your code harder to read and understand.

If you need to use keyword arguments for class attributes, it is best to use them only for optional attributes. For required attributes, it is best to use regular positional arguments.