SPEC 12: Formatting mathematical expressions

spec-0012/index.md

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+---
+title: "SPEC 12 — Formatting mathematical expressions"
+number: 12
+date: 2024-06-06
+author:
+  - "Pamphile Roy <[email protected]>"
+  - "Matt Haberland <[email protected]>"
+discussion: https://discuss.scientific-python.org/t/spec-12-formatting-mathematical-expressions
+endorsed-by:
+---
+
+## Description
+
+[PEP 8](https://peps.python.org/pep-0008)
+and other established styling documents either
+
+- lack comprehensive guidelines about mathematical expressions, or
+- provide simple rules that ignore the relationship between formatting and readability.
+
+In practice, this leads to varying, even conflicting, mathematical expression
+styles across the ecosystem. We seek to standardize the representation of
+mathematical code for the same reason we standardize formatting of other code:
+it brings consistency to the ecosystem and allows collaborators to focus on
+more important aspects of their work.
+
+## Implementation
+
+These rules are intended to respect and
+complement the [PEP 8 standards](https://peps.python.org/pep-0008), such as using
+[implied line continuation](https://peps.python.org/pep-0008/#maximum-line-length) and
+and [breaking lines before binary operators](https://peps.python.org/pep-0008/#should-a-line-break-before-or-after-a-binary-operator)[^1].
+
+0. Unless otherwise specified, rely on the implicit order of operations;
+   i.e., do not add extraneous parentheses. For example, prefer `u**v + y**z`
+   over `(u**v) + (y**z)`, and prefer `x + y + z` over `(x + y) + z`. A full
+   list of implicit operator priority levels is given by
+   [Operator Precedence](https://docs.python.org/3/reference/expressions.html#operator-precedence).
+1. Always use the `**` operator and unary `+`, `-`, and `~` operators _without_
+   surrounding whitespace. For example, prefer `-x**4` over `- (x ** 4)`.
+2. Always surround non-PEMDAS[^2] operators with whitespace, and always make the priority of
+   non-PEMDAS operators explicit. For example, prefer `(x == y) or (w == t)` over
+   `x==y or w==t`.[^3]
+3. Always surround AS[^2] operators with whitespace.
+4. Typically, surround MD[^2] operators with whitespace, except in the following situations.
+   - When there are lower-priority operators (namely AS) within the same compound
+     expression[^4]. For example, prefer `z = -x * y**t` over `z = -x*y**t`, but
+     prefer `z = w - x*y**t` over `z = w - x * y**t` due to the presence of the
+     lower-priority addition operator.
+   - When the division operation would be written mathematically as a fraction with a
+     horizontal bar. For example, prefer `z = t/v * x/y` over `z = t / v * x / y`
+     if this would be written mathematically as the product of two fractions,
+     e.g. $\frac{t}{v} \cdot \frac{x}{y}$.
+5. Considering the previous rules, only `**`, `*`, `/`, and the unary `+`, `-`, and `~`
+   operators can appear in implicit subexpressions[^4] without spaces. In such expressions,
+
+   - Use at most one unary operator, and if used, ensure that it is the leftmost operator.
+   - Use at most one `**` operator, and if used, ensure that it is the rightmost operator.
+   - Use at most one `/` operator, and if used, ensure that it is the rightmost operator except for `**`.
+
+   To achieve these goals, simplification or the addition of parentheses may be required.
+   For example:
+
+   - The expressions `--x` and `-~x` would be implicit subexpressions without spaces
+     containing more than one unary operator. The former can be simplified to `+x` or
+     simply `x`, and the latter requires explicit parentheses, i.e. `-(~x)`.
+   - The expression `x**y**z` would be an implicit subexpression without spaces
+     containing more than one `**` operator. This code would be executed as `x**(y**z)`
+     following the implicit order, but the explicit parentheses should be included for
+     clarity.
+   - In the expression `t**v*x**y + z`, no spaces are used around the multiplication
+     operator due to the presence of the lower-priority addition operator. However,
+     this would lead to `t**v*x**y` being an implicit subexpression without spaces
+     containing more than one `**` operator. This code would be executed as
+     `(t**v)*(x**y) + z`, but the explicit parentheses should be included for clarity.
+   - In the expression `z + x**y/w`, no spaces are used around the division operator
+     due to the presence of the lower-priority addition operator. However, this would
+     lead to `x**y/w` being an implicit subexpression without spaces containing `**`
+     to the left of another operator. This code would be executed as `z + (x**y)/w`,
+     but the explicit parentheses should be included for clarity.
+
+6. Simplify combinations of unary and binary `+` and `-` operators when possible.
+   For example,
+   - prefer `x + y` over `x + +y`,
+   - prefer `x + y` over `x - -y`,
+   - prefer `x - y` over `x - +y`, and
+   - prefer `x - y` over `x + -y`.
+7. If required to satisfy other style requirements, include line breaks before
+   the outermost explicit subexpression possible. For example, if
+   `t + (w + (x + (y + z))))` must be broken, prefer
+   ```python3
+   (t
+    + (w + (x + (y + z)))))
+   ```
+   over
+   ```python3
+   (t + (w + (x + (y
+                   + z)))))
+   ```
+   If there are multiple candidates, include the break at the first opportunity.
+8. If line breaks must occur within a compound subexpression, the break should
+   be placed before the operator with lowest priority. For example, if
+   (x + y*z) must be broken, prefer
+   ```python3
+   (x
+    + y*z)
+   ```
+   over
+   ```python3
+   (x + y
+    * z)
+   ```
+   If there are multiple candidates, include the break at the first opportunity.
+9. Any of the preceding rules may be broken if there is a clear reason to do so.
+   - _Conflict with other style rules_. For example, there is not supposed to be
+     whitespace surrounding the `**` operator, but one can imagine a chain of `**`
+     operations that exhausts the character limit of a line.
+   - _Domain knowledge_. For instance, in the expression
+     `t = (x + y) - z`, it may be important to emphasize that the addition should be
+     performed first for numerical reasons or because `(x + y)` is a conceptually
+     important quantity. In such cases, consider adding a comment, e.g.
+     ```python3
+     t = (x + y) - z  # perform `x + y` first for precision
+     ```
+     or breaking the expressions into separate logical lines, e.g.
+     ```python3
+     w = x + y
+     t = w - z
+     ```
+
+## Terminology
+
+An "explicit" expression is a code expression enclosed within parentheses or
+otherwise syntactically separated from other expressions (i.e. by code other
+than operators, whitespace, literals, or variables). For example, in the list
+comprehension:
+
+```python3
+[j for j in range(1, i + 1)]
+```
+
+The output expression `j` is one explicit expression and the input sequence
+`range(1, i + 1)` is another.
+
+A "subexpression" is subset of an expression that is either explicit or could
+be made explicit (i.e. with parentheses) without affecting the order of
+operations. In the example above, `j` and `range(1, i + 1)` can also be
+referred to as explicit subexpressions of the whole expression, and `1` and
+`i + 1` are explicit subexpressions of the expression `range(1, i + 1)`. `i` and
+`1` are "implicit" subexpressions of `i + 1`: they could be written as explicit
+subexpressions `(i)` and `(1)` without affecting the order of operations, but they
+are not explicit as written.
+
+As another example, in `x + y*z`, `y*z` is a subexpression because it could be made
+explicit as in `x + (y*z)` without changing the order of operations. However, `x + y`
+would not be a subexpression because `(x + y)*z` would change the order of operations.
+Note that `x + y*z` as a whole may also be referred to as a "subexpression" rather than
+an "expression" even though `(x + y*z)` is not a proper subset of the whole.
+
+A "simple" expression is an expression involving only one operator priority level
+without considering the operators within explicit subexpressions.
+A "compound" expression is an expression involving more than one operator
+priority level without considering the contents of explicit subexpressions.
+For example,
+
+- `x + y - z` is a simple expression because `+` and `-` have the
+  same priority level. There are no explicit subexpressions to be ignored.
+- `x * (y + z)` is also a simple expression because there is only one operator
+  between `x` and the explicit subexpression `(y + z)`; we ignore the contents - and
+  especially the operator - within the explicit subexpression; conceptually, it may
+  regarded as `(...)`.
+- `x*y + z` is a compound expression; there are two operators and no explicit
+  subexpressions that can be ignored.
+
+[^1]:
+    Although examples do not show the use of hanging indent, any of the indentation styles
+    allowed by [PEP 8 Indentation](https://peps.python.org/pep-0008/#indentation) are permitted
+    by this SPEC.
+
+[^2]:
+    The acronym PEMDAS commonly refers to "parentheses", "exponentiation", "multiplication",
+    "division", "addition", and "subtraction". Herein, we will consider these operators
+    to be "PEMDAS operators", and we will also include the unary `+`, `-`, and `~` in
+    this category for convenience. The order of operations of PEMDAS operators is typically
+    taught in primary school and reinforced throughout a programmer's training and
+    experience, so it is assumed that most programmers are comfortable relying on the
+    implicit order of operations of expressions involving a few PEMDAS operations. Implicit
+    order of operations becomes less obvious as the number of distinct operator priority
+    levels increases and when multiple non-PEMDAS operators are involved. Portions of this
+    acronym, namely MD and AS, will be used to refer to the corresponding operators.
+
+[^3]:
+    There is a case for simply eliminating spaces to reinforce the implicit order
+    of operations, as in `x==y or w==t`. However, if this were the rule, following
+    the rule would require users to remember the full order of operations hierarchy
+    and apply it without mistakes. Use of explicit parentheses with non-PEMDAS
+    operators leads to simpler rules, is more explicit, and is not uncommon in
+    existing code.
+
+[^4]:
+    For definitions of "explicit"/"implicit" and "simple"/"compound"
+    "expressions"/"subexpressions", see Terminology.