64 lines
1.8 KiB
Markdown
64 lines
1.8 KiB
Markdown
---
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title: "Python Float Formatting — Equality Trap with Percentages"
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tags: [python, float, formatting, gotcha]
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created: 2026-05-18
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sources: daily/2026-05-18.md
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---
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# Python Float Formatting — Equality Trap with Percentages
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## The Problem
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Binary floating-point arithmetic breaks integer-equality checks on values that look whole:
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```python
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1.1 * 10 # → 11.000000000000002
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(1.1 * 10) % 1 # → 2e-15 (not 0.0!)
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(1.1 * 10) % 1 == 0 # → False ← BUG
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```
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A common context where this bites: formatting percentage strings.
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```python
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def fmt_pct(value):
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"""Format 1.1 → '1.1%', 2.0 → '2%'"""
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pct = value * 100
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if pct % 1 == 0: # WRONG — floats lie here
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return f"{int(pct)}%" # never reached for 1.1
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return f"{pct:.2f}%" # outputs "1.10%" instead of "1.1%"
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```
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**Symptom:** Values like `1.1`, `3.3`, `6.6` produce `"1.10%"`, `"3.30%"`, `"6.60%"` instead of `"1.1%"`, `"3.3%"`, `"6.6%"`.
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## Root Cause
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IEEE 754 double precision cannot represent most decimal fractions exactly. Multiplying by 100 amplifies the error. The `% 1 == 0` check compares against exact zero — which the fractional residue never is.
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## Fix
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Format to a fixed number of decimal places first, then strip trailing zeros:
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```python
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def fmt_pct(value):
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pct = value * 100
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formatted = f"{pct:.10f}".rstrip("0").rstrip(".")
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return f"{formatted}%"
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```
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Or use `Decimal` for exact arithmetic when the source is user-entered text:
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```python
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from decimal import Decimal
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def fmt_pct(value: str) -> str:
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pct = Decimal(value) * 100
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return f"{pct.normalize():f}%"
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```
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## General Rule
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> Never use `float == integer` or `float % 1 == 0` for "is this whole?" checks.
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> Always format to fixed dp first, then apply string operations.
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## See Also
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- `wiki/concepts/python-iso-z-suffix` — another float/string representation trap
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