Positional argument v.s. keyword argument

Question

Based on this

A positional argument is a name that is not followed by an equal sign (=) and default value.

A keyword argument is followed by an equal sign and an expression that gives its default value.

def rectangleArea(width, height):
    return width * height

print rectangleArea(width=1, height=2)

Question> I assume that both width and height are positional arguments. Then why can we also call it with the keyword argument syntax?

Answer 1

Since Python 3.8 introduced positional-only parameters, this post needed an update.

Positional arguments, keyword arguments, required arguments and optional arguments are often confused. Positional arguments ARE NOT THE SAME AS required arguments, and keywords arguments ARE NOT THE SAME AS optional arguments.

Positional arguments are arguments that can be called by their position in the function call.

Keyword arguments are arguments that can be called by their name.

Required arguments are arguments that must passed to the function.

Optional arguments are arguments that can be not passed to the function. In Python, optional arguments are arguments that have a default value.

• Positional argument that is optional (Python 3.8)

  def f(a=2, /):
      pass
  
  
  f()  # Allowed, argument is optional
  f(1)  # Allowed, it's a positional argument
  f(a=1)  # Error, positional only  argument
  

• Positional argument that is required (Python 3.8)

  def f(a, /):
      pass
  
  
  f()  # Error, argument required
  f(1)  # Allowed, it's a positional argument
  f(a=1)  # Error, positional only argument
  

• Keyword argument that is optional

  def f(*, a=1):
      pass
  
  
  f()  # Allowed
  f(1)  # Error, keyword only argument
  f(a=1)  # Allowed, it's a keyword argument
  

• Keyword argument that is required

  def f(*, a)
      pass
  
  
  f()  # Error, argument required
  f(1)  # Error, keyword only arguments
  f(a=1)  # Allowed, it's a keyword argument
  

• Positional-or-keyword argument that is optional

  def f(a=1)
      pass
  
  
  f()  # Allowed, argument is optional
  f(1)  # Allowed, it's a positional argument
  f(a=1)  # Allowed, it's a keyword argument
  
  # In fact this function is the same as
  def f(/, a=1, *):
      pass
  

• Positional-or-keyword argument that is required

  def f(a):
      pass
  
  
  f()  # Error, argument required
  f(1)  # Allowed, it's a positional argument
  f(a=1)  # Allowed, it's a keyword argument
  
  # In fact this function is the same as
  def f(/, a, *):
      pass
  

Conclusion, an argument can be optional or required but not both at the same time. It can also be positional, keyword or both at the same time.

Python 3.8 introduced positional-only parameters.

def f(positional_argument, /, positional_or_keyword_argument, *, keyword_argument):
    pass

Answer 2

A keyword argument is just a positional argument with a default value. You must specify all arguments that don't have a default value. In other words, keyword arguments are only "optional" because they will be set to their default value if not specifically supplied.

Answer 3

Defining parameters and arguments here could help.

1. Parameter: a named entity in the function/method definition that specifies an argument.
2. Argument: a value passed to a function.

For example,

def my_function(parameter_1, parameter_2):
    pass

my_function(argument_1, argument_2)

Now when you say positional argument, you are talking about arguments, so has nothing to do with the function definition. width and height in your example are positional parameters or keyword parameters (so called positional-or-keyword parameters).

How you are calling/passing the value to the function determines if they are positional arguments or keyword arguments.

rectangleArea(1, 2) # positional arguments
rectangleArea(width=1, height=2) # keyword arguments

The thing not many people know is that you can specify a positional-only parameter by using the / in the parameter list (example from here).

def func(positional_only1, positional_only2, /, positional_or_keyword): ...

Similarly, you can also have keyword-only parameters by using the * character.

def func(positional_or_keyword, *, keyword_only1, keyword_only2): ...

Finally, we also have var-positional and var-keyword (a.k.a *args and **kwargs respectively). Meaning, you can have arbitrary sequence of positional arguments or keyword arguments passed to the function.

Answer 4

Positional arguments can be called either using values in order or by naming each. For example, all three of the following would work the same way:

def rectangleArea(width, height):
    return width * height

print(rectangleArea(1, 2))
print(rectangleArea(width=1, height=2))
print(rectangleArea(height=2, width=1))

Answer 5

positional arguments: arguments passed to a function in correct positional order. below program understand the positional arguments of a function

#positional arguments example
def combine(str1, str2):
#To join str1 and str2 with str3
    str3 = str1 + str2
    print(str3)

#call combine() and pass 2 strings
combine("Well", "come")   #positional arguments 

suppose, we passed 'come' first, 'well' second, then the result will be comewell. also, call the function 3 strings become error.

Answer 6

Understand the keyword arguments of a function.

Keyword arguments are arguments that identify the parameters by their names.

#keyword arguments example: 
def employee(name, Id):
    print("Employee Name: ", name)
    print("Employee Id  : ", Id)
#call employee() and pass 2 arguments
employee(name = "inban", Id = "pay001")
employee(Id = "pay002", name = "karthik") #we can change the order args.

Answer 7

I assume that both width and height are positional arguments. Then why can we also call it with the keyword argument syntax?

To prevent that you can use positional-only arguments:

def rectangleArea(width, height, /):
    return width * height

print rectangleArea(width=1, height=2)

The error message would be as follows:

TypeError: rectangleArea() got some positional-only arguments passed as keyword arguments: 'width, height'