main function in python geeksforgeeks

Note: For more information on how importing works in Python, check out Python import: Advanced Techniques and Tips as well as Absolute vs Relative Imports in Python. Python program with main() method does not run, The last else clause in python does not work. With our online interpreter, you can test and debug your code in real-time, all from your web browser. It's evaluated by the interpreter when the file is imported or when it's executed. Defining the main function in Python programming is a necessity to start the execution of the program as it gets executed only when the program is run directly and not executed when imported as a module. For example, print () function prints the given object to the standard output device (screen) or to the text stream file. '__main__' can improve code clarity and correctness. That is possible in Python as well (specifically for Python 3.5 and above). name '__main__'. that your function will return some value acceptable as an input to Example: Lets consider there are two Files(File1.py and File2.py). Modify your best_practices.py file so that it looks like the code below: In this example code, the first 10 lines of the file have the same content that they had before. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. main functions are often used to create command-line tools by specifying What is the top-level code environment. acknowledge that you have read and understood our, Data Structure & Algorithm Classes (Live), Data Structures & Algorithms in JavaScript, Data Structure & Algorithm-Self Paced(C++/JAVA), Full Stack Development with React & Node JS(Live), Android App Development with Kotlin(Live), Python Backend Development with Django(Live), DevOps Engineering - Planning to Production, GATE CS Original Papers and Official Keys, ISRO CS Original Papers and Official Keys, ISRO CS Syllabus for Scientist/Engineer Exam, Python Language advantages and applications, Download and Install Python 3 Latest Version, Statement, Indentation and Comment in Python, How to assign values to variables in Python and other languages, Taking multiple inputs from user in Python, Difference between == and is operator in Python, Python | Set 3 (Strings, Lists, Tuples, Iterations). main functions are often used to create command-line tools by specifying them as entry points for console scripts. Most commonly, the __main__.py file is used to provide Sometimes, when you are importing from a module, you would like to know whether a particular modules function is being used as an import, or if you are just using the original .py (Python script) file of that module. being isolated and importable elsewhere. Having a main () function allows you to call its functionality if you import the module. What is Random Number Generator in Python and how to use it? Note that if you import the module again without quitting Python, there will be no output. one mentioned below are preferred. In Python, standard library functions are the built-in functions that can be used directly in our program. Built-in library function: These are Standard functions in Python that are available to use. The code block below shows the result of running this file as a script: The output that we can see here is the result of the first print(). Nested functions are able to access variables of the enclosing scope. PyQt5 How to change background color of Main window ? Looking for job perks? Adding EV Charger (100A) in secondary panel (100A) fed off main (200A). How To Best Implement Multiprocessing In Python? What are Important Advantages and Disadvantages Of Python? C++, How can I end a function in Python just like using "return" in c++. Python Tutorial Python Programming For Beginners, Python: Interesting Facts You Need To Know, Top 10 Features of Python You Need to Know, Top 10 Python Applications in the Real World You Need to Know, Python Anaconda Tutorial : Everything You Need To Know, Top 10 Reasons Why You Should Learn Python. Script: A Python script is a file that you intend to execute from the command line to accomplish a task. Remember that the special value of "__main__" for the __name__ variable means the Python interpreter is executing your script and not importing it. In the above example, we have declared x and y two variables inside the main function. -m argument: Python code read by the Python interpreter from standard input: Python code passed to the Python interpreter with the -c argument: In each of these situations, the top-level modules __name__ is set to Now, when the File1.py is imported into File2.py, the value of __name__ changes. They are explained in detail Whether to apply this practice to your code is a judgment call on your part. Curated by the Real Python team. Python MySQL Environment Setup Database Connection Creating New Database Creating Tables Insert Operation Read Operation Update Operation Join Operation Performing Transactions Python MongoDB Python MongoDB Python SQLite Python SQLite Python Questions Python dictionary is a built-in type that supports key-value pairs. Example A variable created outside of a function is global and can be used by anyone: x = 300 def myfunc (): print(x) myfunc () print(x) Try it Yourself A default argument is a parameter that assumes a default value if a value is not provided in the function call for that argument. Let us look at the code snippet below: In the above example, the conditional if statement is going to compare the values of the variable __name__ with the string __main__. This kind of code pattern is very common when you are dealing with files that are to be executed as Python scripts, and/or to be imported in other modules. If you want to reuse functionality from your code, define the logic in functions outside main() and call those functions within main(). As a developer generalist, Bryan does Python from the web to data science and everywhere inbetween. Python Basics: What makes Python so Powerful? So Python code often contains a line like: Using this convention one can have a file define classes and functions for use in other programs, and also include code to evaluate only when the file is called as a standalone script. problem. your program will have an exit code of 1, indicating failure, and the 1. package. It does not matter where the main function is present or it is present or not. Inside the conditional block, you have added four lines of code (lines 12, 13, 14, and 15): Now, run your best_practices.py script from the command line to see how the output will change: First, the output shows the result of the print() call outside of process_data(). In particular, be careful about returning strings from your main function. The main () is considered a unique function for several programming languages, also referred to as the point of execution for a program file. the global variable instead of a local name. This conditional will evaluate to True when __name__ is equal to the string "__main__". This is my file to demonstrate best practices. The variable __name__ tells me which context this file is running in. I am confused because I am trying to compare it to Java. As a result, programmers write all the necessary functional definitions on the top, and finally write this statement at the end, to organize the code. the tutorial section Modules. it, the script code would unintentionally execute as well. Now, what will happen when you execute this file as a script on the command line? Given an array of N integers, you have to find if it is possible to partition the array with following rules: Each element should belong to exactly one partition. Here are four key best practices about main() in Python that you just saw: Put code that takes a long time to run or has other effects on the computer in a function or class, so you can control exactly when that code is executed. 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Fortunately, since the partially populated __main__ Python does not have a defined entry point like Java, C, C++, etc. How are you going to put your newfound skills to use? Given an array A of size N of integers. First, the data is created from read_data_from_web(). On Windows, the command prompt typically looks like the example below: The part before the > may look different, depending on your username. Hence, it is always good to call other functions from within the main() to compose your entire task from smaller sub-tasks, that can execute independently. In this way, we can use the __name__ variable to control the execution of your code. The execution of the code starts from the starting line and goes line by line. In all three of these cases, __name__ has the same value: the string '__main__'. Python interpreter, however, runs each line serially from the top of the file and has no explicit main () function. Instead, those files are kept short, There is also a conditional (or if) statement that checks the value of __name__ and compares it to the string "__main__". Not the answer you're looking for? If the source file is executed as the main program, the interpreter sets the __name__ variable to have a value __main__. Why did US v. Assange skip the court of appeal? How to Display Fibonacci Series in Python? Now, you can run the whole processing pipeline from the command line, as shown below: In the output from this execution, you can see that the Python interpreter executed main(), which executed read_data_from_web(), process_data(), and write_data_to_database(). After creating a function we can call it by using the name of the function followed by parenthesis containing parameters of that particular function. You can use the if __name__ == "__main__" idiom to determine the execution context and conditionally run process_data() only when __name__ is equal to "__main__". English version of Russian proverb "The hedgehogs got pricked, cried, but continued to eat the cactus". top-level environment by checking its own __name__, which allows a common is executed first. Arrays in Python What are Python Arrays and how to use them? Python Seaborn Tutorial: What is Seaborn and How to Use it? Code within this block wont run unless the module is executed in the packages path: However, if the module is executed in the top-level code environment, Do i need to make a new main function for every thing i do? A Computer Science portal for geeks. Dating Structure & Algorithms in Python - Personality Paced. If you put a print statement before the if __name__ line then it will print output every time any other code attempts to import that as a module. When you import this file in an interactive session (or another module), the Python interpreter will perform exactly the same steps as when it executes file as a script. When this is done, pip inserts the function call into a template script, where the return value of main is passed into sys.exit () . Inner functions are used so that they can be protected from everything happening outside the function. A main function is used so the file can be imported into a REPL without running as a script, this is what the if statement does. Share Improve this answer Follow edited May 23, 2017 at 11:47 Community Bot 1 1 answered Oct 28, 2010 at 8:59 johnsyweb 135k 23 188 246 2 Next, your script called process_data() and passed data in for modification. It does not matter where the main function is present or it is present or not. To understand more about python main function, lets have a look at the topics that I will be covering in this article: A function is a block of reusable code that forms the foundation of performing actions in a programming language. A main function is used so the file can be imported into a REPL without running as a script, this is what the, @AlexThornton Does that mean you shouldn't use, @juil What @AlexThornton meant is that calling, It's worth noting that there is nothing special, to Python, in the if, Actually the first line that gets executed is, @MadPhysicist just to be clear: it gets executed first because it's the first line, not because it's, @Nietvoordekat. In this case, you took advantage of reusing your code instead of defining all of the logic in main(). If you have experience in C/C++ or Java then you must be thinking about the return type of the function and data type of arguments. In this case, __name__ will include the string "__main__". Leave a comment below and let us know. Practice | GeeksforGeeks | A computer science portal for geeks Get Hired with GFG Your Next Career Move Starts Here! The popular method for file executes as a python script. This distinction is also discussed in How to Run Your Python Scripts. How you utilize this mechanism python3 echo.py) as it will have if You can find more information about the __main__.py file in How to Publish an Open-Source Python Package to PyPI. earlier exemplifies using the sys.exit(main()) convention. The Python interpreter will execute the from time import sleep and print() lines that are outside the function definition, then it will create the definition of the function called process_data(). Go produce the performance tree, by default, computer trusted on the library lxml. Now lets take a look at the second way that the Python interpreter will execute your code: imports. What is the difference between "main()" and "main(_)" in python? Inheritance In Python With Examples: All You Need To Know. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, you don't need the if, you can write just, @RubenBermudez Bad idea, that defies the whole point. During this process, the python interpreter sets the __name__ value to the module name if the Python source file is imported as a module. Then, you changed the conditional block so that it executes main(). Every module has a name and statements in a module can find out the name of its module. On evaluation to True, it executes main(). 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Python programmers have developed a set of good practices to use when you want to develop reusable code. package. Example 1: Input: N = 5 . Python is a great language for performing data analysis tasks. __name__ has the value 'execution_methods', which is the name of the .py file that Python is importing from. Top-level code is the first user-specified Python module that starts running. The Python REPL is another example of a top-level environment, so anything Upcoming Batches For Python Certification Training Course, To understand more about python main function, lets have, Got a question for us? To demonstrate this, we will use the interactive Python interpreter. The idea is to allow the caller to specify the argument name with values so that the caller does not need to remember the order of parameters. Python For Loop Tutorial With Examples To Practice, While Loop In Python : All You Need To Know. In this example, we will create a simple function to check whether the number passed as an argument to the function is even or odd. You can read more about repr() in the Python documentation. Manually raising (throwing) an exception in Python. Try it out now! __name__ takes on different values depending on how you executed your Python file. At least 1 upper-case and 1 lower-case letter, Minimum 8 characters and Maximum 50 characters. Important Python Data Types You Need to Know, PyCharm Tutorial: Writing Python Code In PyCharm (IDE), Python Visual Studio- Learn How To Make Your First Python Program. like this was imported from a different module, for example to unit test Therefore, the statements are executed, and we get the desired output. Content Discovery initiative April 13 update: Related questions using a Review our technical responses for the 2023 Developer Survey. In most Python programs/scripts, you might see a function definition, followed by a conditional statement that looks like the example shown below: Find out our Python Training in Top Cities/Countries. It essentially serves as a starting point for the execution of a program. The third print() will first print the phrase The value of __name__ is, and then it will print the representation of the __name__ variable using Pythons built-in repr(). Here is an example module that consumes the __main__ namespace: Example usage of this module could be as follows: Now, if we started our program, the result would look like this: The exit code of the program would be 1, indicating an error. In some Python scripts, you may see a function definition and a conditional statement that looks like the example below: In this code, there is a function called main() that prints the phrase Hello World! This is especially useful when you can compose your overall task from several smaller sub-tasks that can execute independently. You can invoke it with python -m venv [directory]. When echo.py is imported, the If a module The details of how you can execute Python from your command line are not that important for the purpose of this article, but you can expand the box below to read more about the differences between the command line on Windows, Linux, and macOS. This function is often called the entry point because it is where execution enters the program. (This happens in the background). Has the cause of a rocket failure ever been mis-identified, such that another launch failed due to the same problem? Complete . The contents of __main__.py typically isnt fenced with Apply the event Trigger on the widgets. The semantics are a bit subtle. This code pattern is quite common in Python files that you want to be executed as a script and imported in another module. The __name__ identifier is bound to the name of any module as it's being imported. When we run a program, the interpreter runs the code sequentially and will not run the main function if imported as a module, but the Main Function gets executed only when it is run as a Python program. This import style can be used when referencing modules within a That means that by adding this code at the end of your module: if __name__ == "__main__": import sys fib(int(sys.argv[1])) Threading In Python: Learn How To Work With Threads In Python. Why is "1000000000000000 in range(1000000000000001)" so fast in Python 3? There are three types of functions in Python namely Built-in function, user-defined functions, and anonymous functions. During the import process, Python executes the statements defined in the specified module (but only the first time you import a module). News difficulty : Medium. Why? Connecting Talent with Opportunities Since 2020: 2500+ Companies | 10,000+ Jobs Offered | 20 LPA Avg Salary Hire with Us Join our Hiring Contest to unlock your potential and get hired by the best! in the standard library. Finally, modified_data is passed into write_data_to_database(). "PMP","PMI", "PMI-ACP" and "PMBOK" are registered marks of the Project Management Institute, Inc. MongoDB, Mongo and the leaf logo are the registered trademarks of MongoDB, Inc. 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This is where using the if __name__ == '__main__' code block comes in How to input multiple values from user in one line in Python? Ethical standards in asking a professor for reviewing a finished manuscript and publishing it together. In addition, main() should contain any code that you want to run when the Python interpreter executes the file. them as entry points for console scripts. The first two print some introductory phrases. For example, consider the below program as follows: Another example demonstrates that the reference link is broken if we assign a new value (inside the function). No matter which way of running your code youre using, Python defines a special variable called __name__ that contains a string whose value depends on how the code is being used. How To Create Your First Python Metaclass? In this way, I put the code I want to run within main(), the programming logic in a function called demo and called main() within the conditional block. handy. 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