gh-149221:Fix binomialvariate Function for random module

[lighting9999]

Fix:
Replace _log2(random()) with _log2(1.0 - random()). This matches the standard algorithm (1.0 - random() is never zero), produces the correct distribution, and eliminates the crash.

• Issue: random module binomialvariate function returns wrong results #149221

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[skirpichev]

I think it's not too hard to find a seed, that trigger an exception. Maybe issue worth a test?

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Documentation build overview

📚 cpython-previews | 🛠️ Build #32508257 | 📁 Comparing 32e5659 against main (be9c7cb)

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30 files changed · ± 30 modified

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• contents.html
• download.html
• genindex-B.html
• genindex-C.html
• genindex-E.html
• genindex-H.html
• genindex-I.html
• genindex-L.html
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• and 20 more...

[rhettinger]

This is all that is needed:

diff --git a/Lib/random.py b/Lib/random.py
index c89cbb755ab..2460ac3d986 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -836,7 +836,11 @@ def binomialvariate(self, n=1, p=0.5):
             if not c:
                 return x
             while True:
-                y += _floor(_log2(random()) / c) + 1
+                try:
+                    y += _floor(_log2(random()) / c) + 1
+                except ValueError:
+                    # Handle rare case of where random() gives 0.0
+                    continue
                 if y > n:
                     return x
                 x += 1

[lighting9999]

This is all that is needed:

diff --git a/Lib/random.py b/Lib/random.py
index c89cbb755ab..2460ac3d986 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -836,7 +836,11 @@ def binomialvariate(self, n=1, p=0.5):
             if not c:
                 return x
             while True:
-                y += _floor(_log2(random()) / c) + 1
+                try:
+                    y += _floor(_log2(random()) / c) + 1
+                except ValueError:
+                    # Handle rare case of where random() gives 0.0
+                    continue
                 if y > n:
                     return x
                 x += 1

What I originally had in mind was this:

u = random()
if u == 0.0:
    continue
y += _floor(_log2(u) / c) + 1

No try-except, just a simple check and skip. @rhettinger

[rhettinger]

I prefer the zero-cost try/except to not slow down the loop.

[lighting9999]

I prefer the zero-cost try/except to not slow down the loop.

Ok wait to change

[lighting9999]

I prefer the zero-cost try/except to not slow down the loop.

please review this @rhettinger.

[lighting9999]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@rhettinger: please review the changes made to this pull request.