What is the problem with reduce()?

by

As Guido says in his The fate of reduce() in Python 3000 post:

So now reduce(). This is actually the one I’ve always hated most, because, apart from a few examples involving + or *, almost every time I see a reduce() call with a non-trivial function argument, I need to grab pen and paper to diagram what’s actually being fed into that function before I understand what the reduce() is supposed to do. So in my mind, the applicability of reduce() is pretty much limited to associative operators, and in all other cases it’s better to write out the accumulation loop explicitly.

There is an excellent example of a confusing reduce in the Functional Programming HOWTO article:

Quick, what’s the following code doing?

total = reduce(lambda a, b: (0, a[1] + b[1]), items)[1]

You can figure it out, but it takes time to disentangle the expression to figure out
what’s going on. Using a short nested def statements makes things a little bit better:

def combine (a, b):
    return 0, a[1] + b[1]

total = reduce(combine, items)[1]

But it would be best of all if I had simply used a for loop:

total = 0
for a, b in items:
    total += b

Or the sum() built-in and a generator expression:

total = sum(b for a,b in items)

Many uses of reduce() are clearer when written as for loops.

More Related Contents:

  1. Can someone please tell me why the two python codes below produce entirely different outcomes?
  2. How do I print a word on a line, and after every second it prints the word with one extra letter on the end of it every time
  3. List and maximum numbers
  4. How do you input integers using input in Python
  5. Reloading module giving NameError: name ‘reload’ is not defined
  6. How to extract text from an existing docx file using python-docx
  7. How to capture output of Python’s interpreter and show in a Text widget?
  8. How can I suppress the newline after a print statement?
  9. Can I move a virtualenv?
  10. How to upload a file to Google Cloud Storage on Python 3?
  11. ValueError: invalid literal for int () with base 10
  12. How to know/change current directory in Python shell?
  13. How to override the pip command to Python3.x instead of Python2.7?
  14. Why is Python 3 not backwards compatible? [closed]
  15. Python 3: How to specify stdin encoding
  16. Adding a data file in Pyinstaller using the onefile option
  17. Subtract a value from every number in a list in Python?
  18. How does a for loop evaluate its argument
  19. Priority of the logical operators (order of operations) for NOT, AND, OR in Python
  20. How to check if an object has an attribute in Python?
  21. Repeated MySQL queries from Python return the same data [duplicate]
  22. Error while downloading the requirements using pip install (setup command: use_2to3 is invalid.)
  23. What does it mean when the parentheses are omitted from a function or method call?
  24. How to implement barriers to stop the player moving through walls
  25. Disable hash randomization from within python program
  26. nargs=* equivalent for options in Click
  27. User input boolean in python
  28. Python : The second for loop is not running
  29. RuntimeError: Input type (torch.FloatTensor) and weight type (torch.cuda.FloatTensor) should be the same
  30. Confusion about pandas copy of slice of dataframe warning

Leave a Comment