Python numpy append() function is used to merge two arrays. This function returns a new array and the original array remains unchanged.

## NumPy append() Syntax

The function syntax is:

```
numpy.append(arr, values, axis=None)
```

- The
**arr**can be an array-like object or a NumPy array. The values are appended to a copy of this array. - The
**values**are array-like objects and it’s appended to the end of the “arr” elements. - The
**axis**specifies the axis along which values are appended. If the axis is not provided, both the arrays are flattened.

## Python numpy.append() Examples

Let’s look at some examples of NumPy append() function.

### 1. Flattening Two Arrays

```
import numpy as np
arr1 = np.array([[1, 2], [3, 4]])
arr2 = np.array([[10, 20], [30, 40]])
# no axis provided, array elements will be flattened
arr_flat = np.append(arr1, arr2)
print(arr_flat) # [ 1 2 3 4 10 20 30 40]
```

### 2. Merging Along Axis

```
import numpy as np
arr_merged = np.append([[1, 2], [3, 4]], [[10, 20], [30, 40]], axis=0)
print(f'Merged 2x2 Arrays along Axis-0:n{arr_merged}')
arr_merged = np.append([[1, 2], [3, 4]], [[10, 20], [30, 40]], axis=1)
print(f'Merged 2x2 Arrays along Axis-1:n{arr_merged}')
```

**Output:**

```
Merged 2x2 Arrays along Axis-0:
[[ 1 2]
[ 3 4]
[10 20]
[30 40]]
Merged 2x2 Arrays along Axis-1:
[[ 1 2 10 20]
[ 3 4 30 40]]
```

- When the 2×2 arrays are merged along the x-axis, the output array size is 4×2.
- When the 2×2 arrays are merged along the y-axis, the output array size is 2×4.

### 3. Merging Arrays of different shapes

The append() function throws ValueError if both the arrays are of different shape, excluding the axis.

Let’s understand this scenario with a simple example.

```
arr3 = np.append([[1, 2]], [[1, 2, 3], [1, 2, 3]])
print(arr3)
arr3 = np.append([[1, 2]], [[1, 2], [3, 4]], axis=0)
print(arr3)
```

- In the first example, the array elements are flattened. So even if they have completely different size – 1×2 and 2×3, the append() works fine.
- In the second example, the array shapes are 1×2 and 2×2. Since we are appending along the 0-axis, the 0-axis shape can be different. The other shapes should be the same, so this append() will also work fine.

**Output:**

```
[1 2 1 2 3 1 2 3]
[[1 2]
[1 2]
[3 4]]
```

Let’s look at another example where ValueError will be raised.

```
>>> import numpy as np
>>>
>>> arr3 = np.append([[1, 2]], [[1, 2, 3]], axis=0)
Traceback (most recent call last):
File "", line 1, in
File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/numpy/lib/function_base.py", line 4528, in append
return concatenate((arr, values), axis=axis)
ValueError: all the input array dimensions except for the concatenation axis must match exactly
>>>
```

The array shapes are 1×2 and 2×3. Since the axis-1 shapes are different, ValueError is raised.

**Reference**: API Doc