python: repeating iterable data structures

Programming_Languages/Python/Syntax/Repeating_iterable_data_structures.md

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+---
+categories:
+  - Programming Languages
+tags: [python]
+---
+
+# Repeating iterable data structures
+
+In JavaScript when we return data from an API we tend to use an array of objects as the canonical form of a repeating iterable, e.g:
+
+```js
+const data = [
+  {
+    name: "Thomas",
+    age: 35,
+  },
+  {
+    name: "Joe",
+    age: 75,
+  },
+];
+```
+
+In Python there are two common ways to handle similar data structures:
+
+- A list of lists:
+  ```py
+  data = [
+    ["Thomas", 35],
+    ["Joe", 75]
+  ]
+  ```
+- A list of dictionaries:
+  ```py
+  data = [
+     {"name": "Thomas", "age": 35},
+     {"name": "Joe", "age": 75},
+  ]
+  ```
+
+## List of lists
+
+### Sorting by common property
+
+Assuming the sub-lists have an identical structure, you can [sort](/Programming_Languages/Python/Syntax/Sorting_lists_in_Python.md) them by a common property by passing a [lambda function](/Programming_Languages/Python/Syntax/Lambdas_in_Python.md) to the `key` value of `sorted()` and `.sort()`.
+
+For example, to sort the following list of lists by the second `age` property:
+
+```python
+people = [["Alice", 30], ["Bob", 25], ["Clare", 35], ["Dave", 28]]
+```
+
+Using `sorted()`:
+
+```py
+sorted_people = sorted(people, key=lambda x: x[1])
+print(sorted_people)
+# Output: [['Bob', 25], ['Dave', 28], ['Alice', 30], ['Clare', 35]]
+```
+
+Using `.sort`:
+
+```py
+people.sort(key=lambda x: x[1])
+print(people)
+# Output: [['Bob', 25], ['Dave', 28], ['Alice', 30], ['Clare', 35]]
+```
+
+If you want to sort by name instead, you could change the lambda function to `lambda x: x[0]`:
+
+```python
+# Using sorted()
+sorted_people = sorted(people, key=lambda x: x[0])
+print(sorted_people)
+# Output: [['Alice', 30], ['Bob', 25], ['Clare', 35], ['Dave', 28]]
+
+# Using .sort()
+people.sort(key=lambda x: x[0])
+print(people)
+# Output: [['Alice', 30], ['Bob', 25], ['Clare', 35], ['Dave', 28]]
+```
+
+### Updating a value within a list of lists
+
+We can use `map` to mutate a given value within each list.
+
+In the following example we have a list of the following structure:
+
+```py
+data = [
+    ["1688582410", "Article One"],
+    ["1688647447", "Article Two"],
+    ["1689023491", "Article Three"],
+]
+```
+
+Below, we apply a function to each of the first elements which is a Unix timestamp, converting it to a readable format:
+
+```py
+ readable_date = list(map(lambda i: [convert_timestamp(i[0])] + i[1:], date))
+```
+
+Key points:
+
+- We apply the `convert_timestamp` function to the first element of each sublist
+- We wrap this first element in `[]` so that it can be merged with the other elements of the list. This is necessary otherwise we will just return a list of the first elements and not include the other properties.
+- The map and lambda is the core structure. We wrap it in `list` because `map` returns an object not a list.