numpy.hstack() in Python Last Updated : 12 Jun, 2025 Comments Improve Suggest changes Like Article Like Report numpy.hstack() function stacks arrays in sequence horizontally (column-wise). It joins arrays along their second axis for 2D arrays or flattens and joins them for 1D arrays. This is useful for combining arrays side by side. Example: Python import numpy as np a = np.array([1, 2, 3]) b = np.array([4, 5, 6]) res = np.hstack((a, b)) print(res) Output[1 2 3 4 5 6] Arrays a and b are horizontally stacked to form one combined 1D array.Syntaxnumpy.hstack(tup, *, dtype=None, casting='same_kind')Parameters:ParameterTypeDescriptiontupsequence of array_likeArrays to stack horizontally (must match in all but the second axis).dtype data-type, optionalDesired data type of the result array.casting{'no', 'equiv', 'safe', 'same_kind', 'unsafe'}, optionalControls data casting (default: 'same_kind').Returns: This function returns horizontally stacked array of the input arrays.ExamplesExample 1: Horizontal stacking of 2D arrays Python import numpy as np a = np.array([[1, 2], [3, 4]]) b = np.array([[5, 6], [7, 8]]) res = np.hstack((a, b)) print(res) Output[[1 2 5 6] [3 4 7 8]] Each row of a and b is concatenated horizontally to form a wider 2D array.Example 2: Stacking arrays of different shapes (raises an error) Python import numpy as np a = np.array([[1, 2]]) b = np.array([[3, 4], [5, 6]]) res = np.hstack((a, b)) OutputValueError: all the input array dimensions except for the concatenation axis must match exactly, but along dimension 0, the array a...Arrays must be compatible in shape except along the concatenation axis.Example 3: Horizontal stacking with negative numbers Python import numpy as np a = np.array([-1, -2, -3]) b = np.array([4, 5, 6]) res = np.hstack((a, b)) print(res) Output[-1 -2 -3 4 5 6] Works with negative integers too. The resulting array preserves the sign of the original numbers. Comment More info J jana_sayantan Follow Improve Article Tags : Python Python-numpy Python numpy-arrayManipulation Explore Python FundamentalsPython Introduction 3 min read Input and Output in Python 4 min read Python Variables 5 min read Python Operators 5 min read Python Keywords 2 min read Python Data Types 7 min read Conditional Statements in Python 3 min read Loops in Python - For, While and Nested Loops 6 min read Python Functions 5 min read Recursion in Python 4 min read Python Lambda Functions 5 min read Python Data StructuresPython String 5 min read Python Lists 4 min read Python Tuples 4 min read Python Dictionary 3 min read Python Sets 6 min read Python Arrays 7 min read List Comprehension in Python 4 min read Advanced PythonPython OOP Concepts 11 min read Python Exception Handling 5 min read File Handling in Python 4 min read Python Database Tutorial 4 min read Python MongoDB Tutorial 2 min read Python MySQL 9 min read Python Packages 12 min read Python Modules 7 min read Python DSA Libraries 15 min read List of Python GUI Library and Packages 3 min read Data Science with PythonNumPy Tutorial - Python Library 3 min read Pandas Tutorial 6 min read Matplotlib Tutorial 5 min read Python Seaborn Tutorial 15+ min read StatsModel Library- Tutorial 4 min read Learning Model Building in Scikit-learn 8 min read TensorFlow Tutorial 2 min read PyTorch Tutorial 6 min read Web Development with PythonFlask Tutorial 8 min read Django Tutorial | Learn Django Framework 7 min read Django ORM - Inserting, Updating & Deleting Data 4 min read Templating With Jinja2 in Flask 6 min read Django Templates 7 min read Python | Build a REST API using Flask 3 min read How to Create a basic API using Django Rest Framework ? 4 min read Python PracticePython Quiz 3 min read Python Coding Practice 1 min read Python Interview Questions and Answers 15+ min read Like