- 5.1 Function Definitions
- 5.2 Default Arguments
- 5.3 Variadic Arguments
- 5.4 Keyword Arguments
- 5.5 Variadic Keyword Arguments
- 5.6 Functions Accepting All Inputs
- 5.7 Positional-Only Arguments
- 5.8 Names, Documentation Strings, and Type Hints
- 5.9 Function Application and Parameter Passing
- 5.10 Return Values
- 5.11 Error Handling
- 5.12 Scoping Rules
- 5.13 Recursion
- 5.14 The lambda Expression
- 5.15 Higher-Order Functions
- 5.16 Argument Passing in Callback Functions
- 5.17 Returning Results from Callbacks
- 5.18 Decorators
- 5.19 Map, Filter, and Reduce
- 5.20 Function Introspection, Attributes, and Signatures
- 5.21 Environment Inspection
- 5.22 Dynamic Code Execution and Creation
- 5.23 Asynchronous Functions and await
- 5.24 Final Words: Thoughts on Functions and Composition
This chapter is from the book
This chapter is from the book
This chapter is from the book
5.7 Positional-Only Arguments
Many of Python’s built-in functions only accept arguments by position. You’ll see this indicated by the presence of a slash (/) in the calling signature of a function shown by various help utilities and IDEs. For example, you might see something like func(x, y, /). This means that all arguments appearing before the slash can only be specified by position. Thus, you could call the function as func(2, 3) but not as func(x=2, y=3). For completeness, this syntax may also be used when defining functions. For example, you can write the following:
def func(x, y, /):
pass
func(1, 2) # Ok
func(1, y=2) # Error
This definition form is rarely found in code since it was first supported only in Python 3.8. However, it can be a useful way to avoid potential name clashes between argument names. For example, consider the following code:
import time
def after(seconds, func, /, *args, **kwargs):
time.sleep(seconds)
return func(*args, **kwargs)
def duration(*, seconds, minutes, hours):
return seconds + 60 * minutes + 3600 * hours
after(5, duration, seconds=20, minutes=3, hours=2)
In this code, seconds is being passed as a keyword argument, but it’s intended to be used with the duration function that’s passed to after(). The use of positional-only arguments in after() prevents a name clash with the seconds argument that appears first.