{"id":2003,"date":"2019-12-30T17:32:58","date_gmt":"2019-12-30T17:32:58","guid":{"rendered":"https:\/\/www.askpython.com\/?p=2003"},"modified":"2023-02-16T19:57:20","modified_gmt":"2023-02-16T19:57:20","slug":"python-setattr-function","status":"publish","type":"post","link":"https:\/\/www.askpython.com\/python\/python-setattr-function","title":{"rendered":"Python setattr() Function"},"content":{"rendered":"\n

Python setattr()<\/code> function is used to set the attribute of an object, given its name. While being a very simple function, it can prove to be very useful in the context of Object Oriented Programming in Python<\/a>. Let us look at how we could use this function in our Python programs.<\/p>\n\n\n\n

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setattr() Function Syntax<\/h2>\n\n\n\n

It takes the object name, the attribute name and the value as the parameters, and sets object.attribute<\/code> to be equal to value<\/code>. Since any object attribute can be of any type, no exception is raised by this function.<\/p>\n\n\n\n

Format<\/strong>: setattr(object, attr, value)<\/code><\/p>\n\n\n\n

Here is a simple example to demonstrate the use of setattr()<\/code>.<\/p>\n\n\n

\nclass MyClass():\n    def __init__(self, name, value):\n        # Set the attribute of the class object\n        setattr(self, name, value)\n\na = MyClass('KEY', 100)\nprint('Printing attribute from Class Object:', a.KEY)\nprint('Printing attribute from getattr():', getattr(a, 'KEY'))\n<\/pre><\/div>\n\n\n
Output<\/strong><\/p>\n\n\n
\nPrinting attribute from Class Object: 100\nPrinting attribute from getattr(): 100\n<\/pre><\/div>\n\n\n
setattr()<\/code> is very useful when the attribute of the object is not know beforehand, and cannot be set using object.attribute_name = value<\/code>.<\/p>\n\n\n\n
This is a very handy method, used whenever the attributes of the object can change during runtime, and demonstrates how Object Oriented Programming still performs well in these scenarios.<\/p>\n\n\n\n
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Using setattr() with getattrr()<\/h2>\n\n\n\n
It is commonly used in tandem with the getattr()<\/code> method, to get and set the attributes of Objects.<\/p>\n\n\n\n
Here is an example to demonstrate some of the use-cases of setattr()<\/code>, when paired with the getattr()<\/code> method.<\/p>\n\n\n\n
This example constructs objects and sets the attributes of each subject to their corresponding marks, for a single student.<\/p>\n\n\n\n
After constructing the Student object using setattr()<\/code>, we use getattr()<\/code> and sort the subject-wise marks of students.<\/p>\n\n\n
\nclass Student():\n\tdef __init__(self, name, results):\n\t\tself.name = name\n\t\tfor key, value in results.items():\n                        # Sets the attribute of the 'subject' to\n                        # the corresponding subject mark.\n                        # For example: a.'Chemistry' = 75\n\t\t\tsetattr(self, key, value)\n\n\tdef update_mark(self, subject, mark):\n\t\tself.subject = mark\n\n\nsubjects = ['Physics', 'Chemistry', 'Biology']\n\na = Student('Amit', {key: value for (key, value) in zip(subjects, [73, 81, 90])})\n\nb = Student('Rahul', {key: value for (key, value) in zip(subjects, [91, 89, 74])})\n\nc = Student('Sunil', {key: value for (key, value) in zip(subjects, [59, 96, 76])})\n\nstudent_list = [a, b, c]\n\nstud_names = [student.name for student in student_list]\n\nprint('Sorted Physics Marks:')\nprint(sorted([getattr(s, 'Physics') for s in student_list]))\n\nprint('\\nSorted Marks for all subjects:')\nprint(sorted([getattr(s, subject) for s in student_list for subject in subjects]))\n\nprint('\\nSorted Marks for every Student:')\nprint(dict(zip(stud_names, [sorted([getattr(s, subject) for subject in subjects]) for s in student_list])))\n<\/pre><\/div>\n\n\n
While some of the Python one-liners may seem very complicated, it is not. The first one sorted([getattr(s, 'Physics') for s in student_list])<\/code> is equivalent to:<\/p>\n\n\n
\nls = []\nfor s in student_list:\n    ls.append(getattr(s, 'Physics'))\n# Sort the list\nls.sort()\nprint(ls)\n<\/pre><\/div>\n\n\n
The second one liner is also extremely similar, but using two nested loops instead of one.<\/p>\n\n\n
\nls = []\nfor s in student_list:\n    for subject in subjects:\n        ls.append(getattr(s, subject))\nls.sort()\nprint(ls)\n<\/pre><\/div>\n\n\n
The last one is a bit tricky, wherein you construct a Dictionary of name: sorted_subject_marks<\/code> for each student.<\/p>\n\n\n\n
We first iterate through each name and get the attribute from the student object list, and then sort the intermediate list, before adding to our Dictionary.<\/p>\n\n\n
\ndct = {}\nfor name, s in zip(subjects, student_list):\n    ls = []\n    for subject in subjects:\n        ls.append(getattr(s, subject))\n    ls.sort()\n    dct[name] = ls\nprint(dct)\n<\/pre><\/div>\n\n\n
Output of the complete code snippet:<\/p>\n\n\n
\nSorted Physics Marks:\n[59, 73, 91]\n\nSorted Marks for all subjects:\n[59, 73, 74, 76, 81, 89, 90, 91, 96]\n\nSorted Marks for every Student:\n{'Amit': [73, 81, 90], 'Rahul': [74, 89, 91], 'Sunil': [59, 76, 96]}\n<\/pre><\/div>\n\n\n
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Conclusion<\/h2>\n\n\n\n
In this article, we learned about the setattr()<\/code> method, which is used to dynamically set the object attributes at runtime. This is a very useful method for Object Oriented Programming, when the attributes are not known to the developer, and is thus used for building flexible API.<\/p>\n\n\n\n
References<\/h2>\n\n\n\n