3 ms·

It only makes sense to swap elements of an arrays (lists). Swapping variable is super rare. I can't recall the same time I had to swap two integers/floats or e

by xxs 5y ago

It only makes sense to swap elements of an arrays (lists). Swapping variable is super rare. I can't recall the same time I had to swap two integers/floats or even object references.

eesmith 5y ago
You're right! I was surprised. I did: ag '(^|[^a-zA-Z0-9=])([a-zA-Z_][a-zA-Z0-9_]*), ?([a-zA-Z_][a-zA-Z0-9_]*) ?= ?\3, ?\2' across several Python code bases I've worked on, and indeed found very few examples. Here's one from Python's statistics.py: X, Y = self, other if (Y._sigma, Y._mu) < (X._sigma, X._mu): # sort to assure commutativity X, Y = Y, X NumPy has only one match that isn't in test code, in numpy/core/numeric.py:convolve if (len(v) > len(a)): a, v = v, a It's a bit more common in SciPy, like scipy/linalg/blas.py:_get_funcs which has: if prefer_fortran: module1, module2 = module2, module1 but even then, there are only 26 relevant matches across 421KLoC.
- dotancohen 5y ago
  > ag '(^|[^a-zA-Z0-9=])([a-zA-Z_][a-zA-Z0-9_]*), ?([a-zA-Z_][a-zA-Z0-9_]*) ?= ?\3, ?\2' Why are you checking `(^|[^a-zA-Z0-9=])` before the first variable? Why are you storing it? Wouldn't `^\s*` (beginning of line, then 0 or more whitespace, not stored) be simpler? I ask to learn, not critically.
  - eesmith 5y ago
    The exclusion test is to reduce false positives like: return open(filename, mode, encoding=encoding, errors=errors) ^^^^^^^^^^^^^^^^^^^^^^^ This case matches because of the under-caret'ed e, encoding=encoding, e You are right that requiring only leading whitespace would be clearer. I didn't think of it. I only wanted to eliminate those false positives I was seeing with my original test, which only had \2, \1. In thinking more about it, your whitespace-only gives a false negative for constructs like: progressbar/widgets.py:306: if not self.fill_left: rpad, lpad = lpad, rpad matplotlib/lib/matplotlib/widgets.py:1093: if vmin>vmax: vmin, vmax = vmax, vmin The reason I stored \1 was simply to get the grouping. I didn't recall the syntax for non-capturing groups, and it wasn't worthwhile to look up.
    - dotancohen 5y ago
      I see, thank you. Targeting the if statements, but not the open() methods, is an interesting corner case that I would not have though about.
      - eesmith 5y ago
        I did mine because I thought some people might write it: (a, b) = (b, a) so the grouping stands out better. Turns out I didn't find any examples of that.
jasonhansel 5y ago
It's a bit more common in C when swapping pointers, since it can help you ensure that there's only one pointer to a given structure at any time, and it can simplify memory management in certain cases by letting you repurpose an existing allocation.

by xxs 5y ago

It only makes sense to swap elements of an arrays (lists). Swapping variable is super rare. I can't recall the same time I had to swap two integers/floats or even object references.

eesmith 5y ago

You're right! I was surprised. I did: ag '(^|[^a-zA-Z0-9=])([a-zA-Z_][a-zA-Z0-9_]*), ?([a-zA-Z_][a-zA-Z0-9_]*) ?= ?\3, ?\2' across several Python code bases I've worked on, and indeed found very few examples. Here's one from Python's statistics.py: X, Y = self, other if (Y._sigma, Y._mu) < (X._sigma, X._mu): # sort to assure commutativity X, Y = Y, X NumPy has only one match that isn't in test code, in numpy/core/numeric.py:convolve if (len(v) > len(a)): a, v = v, a It's a bit more common in SciPy, like scipy/linalg/blas.py:_get_funcs which has: if prefer_fortran: module1, module2 = module2, module1 but even then, there are only 26 relevant matches across 421KLoC.

dotancohen 5y ago
> ag '(^|[^a-zA-Z0-9=])([a-zA-Z_][a-zA-Z0-9_]*), ?([a-zA-Z_][a-zA-Z0-9_]*) ?= ?\3, ?\2' Why are you checking `(^|[^a-zA-Z0-9=])` before the first variable? Why are you storing it? Wouldn't `^\s*` (beginning of line, then 0 or more whitespace, not stored) be simpler? I ask to learn, not critically.
- eesmith 5y ago
  The exclusion test is to reduce false positives like: return open(filename, mode, encoding=encoding, errors=errors) ^^^^^^^^^^^^^^^^^^^^^^^ This case matches because of the under-caret'ed e, encoding=encoding, e You are right that requiring only leading whitespace would be clearer. I didn't think of it. I only wanted to eliminate those false positives I was seeing with my original test, which only had \2, \1. In thinking more about it, your whitespace-only gives a false negative for constructs like: progressbar/widgets.py:306: if not self.fill_left: rpad, lpad = lpad, rpad matplotlib/lib/matplotlib/widgets.py:1093: if vmin>vmax: vmin, vmax = vmax, vmin The reason I stored \1 was simply to get the grouping. I didn't recall the syntax for non-capturing groups, and it wasn't worthwhile to look up.
  - dotancohen 5y ago
    I see, thank you. Targeting the if statements, but not the open() methods, is an interesting corner case that I would not have though about.
    - eesmith 5y ago
      I did mine because I thought some people might write it: (a, b) = (b, a) so the grouping stands out better. Turns out I didn't find any examples of that.

jasonhansel 5y ago

It's a bit more common in C when swapping pointers, since it can help you ensure that there's only one pointer to a given structure at any time, and it can simplify memory management in certain cases by letting you repurpose an existing allocation.