Python: Keyword Argument Default Value Unstable

By Xah Lee. Date: . Last updated: .

Keyword argument's default value is unstable.

Example:

# -*- coding: utf-8 -*-
# python 2

def f(xx, yy=[]):
    yy.append(xx)
    return yy

print f(1)  # [1]
print f(1)  # [1, 1]

this example is from the official Python tutorial. 4. More Control Flow Tools — Python v2.7.6 documentation#default-argument-values

explained as follows:

Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes.

solution:

# -*- coding: utf-8 -*-
# python 2

def f(xx, yy=None):
    if yy is None:
        yy = []
    yy.append(xx)
    return yy

print f(1)  # [1]
print f(1)  # [1]

Python Semantic Bad

  1. Why List Comprehension is Bad
  2. Python 3: Map with Side Effect Doesn't Work If Result is Not Used
  3. The Idiocy of Python's Function Parameter Specification
  4. Python Scope Complexity, Shallow Copy, Deep Copy, Circular List, and the Garbage Underneath Computer Languages
  5. Python: What's the Difference Between 「dict={}」 vs 「dict.clear()」?
  6. Python: Get Number of Arguments of Function
  7. Python: Copy Nested List, Shallow/Deep Copy
  8. Python: Keyword Argument Default Value Unstable
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  10. FCK Python: String Methods, Functions, Slashes and Backslashes
  11. Python Bitwise Invert Oddity
  12. From Why Not Ruby to FCK Python, Hello Ruby
  13. Why Learn Lisp When There Are Perl and Python

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