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# Aug 19, 2018

## Python Data Structures

### `list.append(x)`

Add an item to the **end** of the list. Equivalent to `a[len(a):] = [x]`.

### `list.extend(iterable )`

Extend the list by **appending all the items from the iterable**. Equivalent to `a[len(a):] = iterable`

```java
List<Intger> list1 = new ArrayList<>();
List<Intger> list2 = new ArrayList<>(4);

// something like list1.extend(list2) in Python
list1.addAll(list2);
```

### `list.insert(i, x)`

Insert an item at a given position. The **first argument is the index of the element before which to insert**, so `a.insert(0, x)` inserts at the front of the list, and `a.insert(len(a), x)` is equivalent to `a.append(x)`.

### `list.remove(x)`

Remove the **first item from the list whose value is equal to** `x`. It raises a `ValueError` if there is no such item.

### `list.pop([i])`

**Remove the item at the given position in the list, and return it**. If **no index is specified**, `a.pop()`**removes and returns the last item in the list**. (The square brackets around the `i` in the method signature denote that the parameter is optional, not that you should type square brackets at that position. You will see this notation frequently in the Python Library Reference.)

### `list.clear()`

**Remove all items from the list**. Equivalent to `del a[:]`.

### `list.index(x[, start[, end]])`

**Return zero-based index in the list of the first item whose value is equal to `x`**. Raises a [`ValueError`](https://docs.python.org/3/library/exceptions.html#ValueError) if there is no such item.

The optional arguments `start` and `end` are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The **returned index is computed relative to the beginning of the full sequence** rather than the `start` argument.

### `list.count(x)`

**Return the number of times** `x` **appears in the list**.

### `list.sort(key=None, reverse=False)`

**Sort the items of the list in place** (the arguments can be used for sort customization, see [`sorted()`](https://docs.python.org/3/library/functions.html#sorted)for their explanation).

### `list.reverse()`

**Reverse the elements of the list in place**.

### `list.copy()`

**Return a shallow copy of the list**. Equivalent to `a[:]`.

## [How do I check if a list is empty?](https://stackoverflow.com/questions/53513/how-do-i-check-if-a-list-is-empty)

```python
if not a:
    print("List is empty")
```

Using the implicit booleanness of the empty list is quite pythonic.