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+---
+title: "SPEC 12 — Formatting mathematical expressions"
+number: 12
+date: 2024-06-06
+author:
+  - "Pamphile Roy <roy.pamphile@gmail.com>"
+discussion: https://discuss.scientific-python.org/t/spec-12-formatting-mathematical-expressions
+endorsed-by:
+---
+
+## Description
+
+It is known that the PEP8 and other established styling documents are missing
+guidelines about mathematical expressions. This leads to people coming with
+their own interpretation and style. Standardizing the way we represent maths
+would lead to the same benefits seen with "normal" code. It brings consistency
+in the ecosystem improving the collaborative efforts.
+
+This SPEC standardize the formatting of mathematical expressions.
+
+## Implementation
+
+The following rules must be followed.
+These rules respect and complement the PEP8 (relevant sections includes
+[id20](https://www.python.org/dev/peps/pep-0008/#id20) and
+[id20](https://www.python.org/dev/peps/pep-0008/#id28)).
+
+We define a _group_ as a collection of operators having the same priority.
+e.g. `a + b + c` is a single group, `a + b * c` is composed of two groups `a`
+and `b * c`. A group is also a collection delimited with parenthesis.
+`(a + b * c)` is a group. And the whole expression by itself is a
+group. 
+
+- There a space before and after `-` and `+`. Except if
+  the operator is used to define the sign of the number;
+
+  ```
+  a + b
+  -a
+  ```
+
+- Within a group, if operators with different priorities are used, add whitespace around the operators with the lowest priority(ies).
+
+  ```
+  a + b*c
+  ```
+
+- There is no space before and after `**`.
+
+  ```
+  a**b
+  ```
+
+- There is no space before and after operators `*` and `/`. Only exception is if the expression consist of a single operator linking two groups with more than one
+ element.
+
+  ```
+  a*b
+  (a*b) * (c*d)
+  ```
+
+- Operators within a group are ordered from the lowest to the highest priority.
+  If this is technically an issue (e.g. restriction on the AST), add
+  parenthesis or spaces.
+
+  ```
+  a/d*b**c
+  a*(b**c)/d
+  a*b**c / d
+  a * b**c / d
+  ```
+
+- When splitting an equation, new lines should start with the operator linking
+  the previous and next logical block. Single digit on a line are forbidden.
+
+  ```
+  (
+      a/b
+      + c*d
+  )
+  ```
+
+### Examples
+
+```python
+# good
+i = i + 1
+submitted += 1
+x = x*2 - 1
+hypot2 = x*x + y*y
+c = (a + b) * (a - b)
+dfdx = sign*(-2*x + 2*y + 2)
+result = 2*x**2 + 3*x**(2/3)
+y = 4*x**2 + 2*x + 1
+c_i1j = (
+    1./n**2.
+    *np.prod(
+        0.5*(2. + abs(z_ij[i1, :]) + abs(z_ij) - abs(z_ij[i1, :] - z_ij)), axis=1
+    )
+)
+```
+
+```python
+# bad
+i = i + 1
+submitted += 1
+x = x * 2 - 1
+hypot2 = x * x + y * y
+c = (a + b) * (a - b)
+dfdx = sign * (-2 * x + 2 * y + 2)
+result = 2 * x ** 2 + 3 * x ** (2 / 3)
+y = 4 * x ** 2 + 2 * x + 1
+c_i1j = (
+    1.0
+    / n ** 2.0
+    * np.prod(
+        0.5 * (2.0 + abs(z_ij[i1, :]) + abs(z_ij) - abs(z_ij[i1, :] - z_ij)), axis=1
+    )
+)
+```
+
+## Notes
+
+These formatting rules do not make any consideration in terms of performances
+nor precision. The scope is limited to styling.