__name__ has the value 'execution_methods', which is the name of the .py file that Python is importing from. After that, the value of data is printed. But, we want to execute the main function only when the script is executed directly. By contrast, Python does not have a special function that serves as the entry point to a script. 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. 06:36 Although Python does not assign any significance to a function called main(), the best practice here is to name the entry point function main() anyways. But if it’s directly part of the code then it will be executed when the file is imported as a module. Python programmers have developed a set of good practices to use when you want to develop reusable code. On Linux and macOS, the command line typically looks like the example below: The part before the dollar sign ($) may look different, depending on your username and your computer’s name. A module can define functions, classes, and variables. 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! Importing from time and defining process_data() produce no output, just like when you executed the code from the command line. ", "The variable __name__ tells me which context this file is running in.". Global variables are available from within any scope, global and local. Then, you changed the conditional block so that it executes main(). You can find more information about the __main__.py file in How to Publish an Open-Source Python Package to PyPI. Sometimes the code you write will have side effects that you want the user to control, such as: In these cases, you want the user to control triggering the execution of this code, rather than letting the Python interpreter execute the code when it imports your module. This special function is called main. You can easily define a main() function and call it just like you have done with all of the other functions above: A Python Array is a collection of common type of data structures having... How to Use Python Online Compiler Follow the simple steps below to compile and execute Python... Calendar module in Python has the calendar class that allows the calculations for various task... Python vs RUBY vs PHP vs TCL vs PERL vs JAVA. When we run a python program, it executes all the statements inside it. The team members who worked on this tutorial are: Master Real-World Python Skills With Unlimited Access to Real Python. How to Write a Python main Function? Add the code below to the bottom of your best_practices.py file: In this code, you’ve added a conditional statement that checks the value of __name__. This has the benefit of meaning that you can loop through data to reach a result. By the end of this article, you’ll understand: Free Download: Get a sample chapter from Python Tricks: The Book that shows you Python’s best practices with simple examples you can apply instantly to write more beautiful + Pythonic code. Specifically, the outputs of the calls to print() that are inside the definition of process_data() are not printed! Once the Python interpreter imports the file, you can use any variables, classes, or functions defined in the module you’ve imported. When you are developing a module or script, you will most likely want to take advantage of modules that someone else has already built, which you can do with the import keyword. These will apply whenever you want to write code that you can run as a script and import in another module or an interactive session. Now, you can start the program. That way, any other programmers who read your script immediately know that this function is the starting point of the code that accomplishes the primary task of the script. Note that if you import the module again without quitting Python, there will be no output. You can read more about conditional statements in Conditional Statements in Python. Inside the function body, the return statement determines the value to be returned. The remedy is to let main()'s return value specify the exit status. This distinction is also discussed in How to Run Your Python Scripts. This function is usually called main() and must have a specific return type and arguments according to the language standard. So, if we have a main() function and we call it in the program directly, it will get executed all the time, even when the script is imported as a module. best-practices 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 Python’s built-in repr(). 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(). Defining the main function in Python is not mandatory but it is a good practice to do so for better readability of your program. This is my file to demonstrate best practices. To understand this, consider the following example code. In general, all the programming languages have main() function as a pre-defined function/ method to indicate to the compiler that it is the beginning of the execution flow. Reasons to have that if statement calling main() (in no particular order): Other languages (like C and Java) have a main() function that is called when the program is executed. However, there are slight differences in meaning that emphasize the purpose of a piece of code: File: Typically, a Python file is any file that contains code. Python main function. The module is a program that can be included or imported to the other programs so that it can be reused in the future without writing the same m… You can see this in the third line of output above. However, including a main() function in your Python program can be handy to structure your code logically - all of the most important components are contained within this main() function. A program gets executed only when it is executed from the main function and it does not get executed when it is imported as a module. Main function is executed only when it is run as a Python program. During the import process, Python executes the statements defined in the specified module (but only the first time you import a module). We will have a Python class. If you are familiar with other programming languages like C++ and Java there you must have seen the main() function, that executes first before any other code statement when the code is compiled. This is better than putting the code directly into the conditional block because a user can reuse main()if they import your module. The standard name of the 'main function' should be __main__. Bryan is a mechanical engineering professor and a core developer of Cantera, the open-source platform for thermodynamics, chemical kinetics, and transport. On Windows, the command prompt typically looks like the example below: The part before the > may look different, depending on your username. XML stands for eXtensible Markup Language. When a module is invoked on the command line, such as: python mymodule.py then the module behaves as though the following lines existed at the end of the module (except that the attribute __sys may not be used or assumed to exist elsewhere in the script): It will not run the main function if it imported as a module. First of all we have to write some code to unit test them. Share The first two print some introductory phrases. Python is a very interesting programming language to learn. An Azure Function should be a stateless method in your Python script that processes input and produces output. Before executing code, Python interpreter reads source file and define few special variables/global variables. The commands that you type will go after the $. Regardless of your operating system, the output from the Python scripts that you use in this article will be the same, so only the Linux and macOS style of input is shown in this article, and the input line will start at the $. Today, in … Technical detail: The Python documentation defines specifically when __name__ will have the value '__main__': A module’s __name__ is set equal to '__main__' when read from standard input, a script, or from an interactive prompt. What is the def main() function in Python? The way that you tell the computer to execute code from the command line is slightly different depending on your operating system. Recursion is a common mathematical and programming concept. You can read more about modules in Python Modules and Packages – An Introduction. intermediate The last two lines of the script are the conditional block that checks __name__ and runs main() if the if statement is True. when the Python interpreter executes it. Then, you stored data from a file in data instead of reading the data from the Web. In all three of these cases, __name__ has the same value: the string '__main__'. When Python interpreter reads a source file, it will execute all the code found in it. The main function in Python acts as a point of execution for any program. This is my file to test Python's execution methods. There is also a conditional (or if) statement that checks the value of __name__ and compares it to the string "__main__". When you execute the main function in python, it will then read the "if" statement and checks whether __name__ does equal to __main__. So when the interpreter runs a module, the __name__ variable will be set as __main__ if the module that is being run is the main program. Main function is the entry point of any program. Python is one of the most popular programming languages to learn. This is especially useful when you can compose your overall task from several smaller sub-tasks that can execute independently. Python programmers have come up with several conventions to define this starting point. Here is how to call main() when the program is executed: if __name__ == '__main__': main() Save the program as "hello.py" (without the quotes). Functions are one of the "first-class citizens" of Python, which means that functions are at the same level as other Python objects like integers, strings, modules, etc. You can call a Python function at the bottom of your program and it will run. Now, you can start the program. Many languages, such as C, C++, Java, and several others, define a special function that must be called main() that the operating system automatically calls when it executes the compiled program. By default, the runtime expects the method to be implemented as a global method called main() in the __init__.py file.You can change the default configuration by specifying the scriptFile and entryPoint properties in the function.json file. The main function in Python acts as the point of execution for any program. Introduction to Python Main function. If your program has if __name__ == “__main__” statement then the program is executed as a standalone program. It will not run the main function if it imported as a module. Notice that importing from time and defining process_data() produce no output. But python interpreter executes the source file code sequentially and doesn’t call any method if it’s not part of the code. Including a main() function, though not required, can structure our Python programs in a logical way that puts the most important components of the program into one function. Finally, modified_data is passed into write_data_to_database(). You can actually give the entry point in a Python script any name you want. If you run this code as a script or import it, you will get the same output as in the previous section. To help understand how this code will execute, you should first understand how the Python interpreter sets __name__ depending on how the code is being executed. in the programming console. Introduction to Python Main function. Code will be cleaner, easier to read, and better organized. Then, you reused process_data()and write_data_to_database() from the best_practices.py file. def is the keyword for defining a function. When you run the function def main (): It is because we did not declare the call function "if__name__== "__main__". In this case, you took advantage of reusing your code instead of defining all of the logic in main(). __name__ is stored in the global namespace of the module along with the __doc__, __package__, and other attributes. Most Python files have the extension .py. Python interpreter uses the main function in two ways direct run: __name__=__main__ if statement == True, and the script in _main_will be executed import as a module __name__= module's filename if statement == false, and the script in __main__ will not be executed When the code is executed, it will check for the module name with "if." This happened because the variable __name__ has the value "__main__" when the Python interpreter executes the file as a script, so the conditional statement evaluated to True. Python also has a main().In Python, the focal point of the execution of any program is the main(), and it acts as the entry point of the code.It is important to define the main function in Python … So, if we have a main() function and we call it in the program directly, it will get executed all the time, even when the script is imported as a module. Mark as Completed Above examples are Python 3 codes, if you want to use Python 2, please consider following code, In Python 3, you do not need to use if__name. If you want to reuse functionality from your code, define the logic in functions outside main() and call those functions within main(). Python main function is a starting point of any program. Import the Best Practices File in Another Module or the Interactive Interpreter. As repr(), return a string containing a printable representation of an object, but escape the non-ASCII characters in the string returned by repr() using \x, \u or \U escapes. Curated by the Real Python team. When you execute a script, you will not be able to interactively define the code that the Python interpreter is executing. This is because the __name__ variable had the value "best_practices", so Python did not execute the code inside the block, including process_data(), because the conditional statement evaluated to False. Then, the script will exit without doing anything further, because the script does not have any code that executes process_data(). The main purpose of the class is to store and retrieve person’s name. 2. Another common practice in Python is to have main() execute other functions, rather than including the task-accomplishing code in main(). In general, all the programming languages have main() function as a pre-defined function/ method to indicate to the compiler that it is the beginning of the execution flow. Module: A Python module is a file that you intend to import from within another module or a script, or from the interactive interpreter. 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(). Function blocks begin with the keyword deffollowed by the function name and parentheses ( ( ) ). The example below demonstrates this situation: Notice that you get the same behavior as before you added the conditional statement to the end of the file! My data read from the Web that has been modified, Importing Into a Module or the Interactive Interpreter, Use if __name__ == "__main__" to Control the Execution of Your Code, Create a Function Called main() to Contain the Code You Want to Run, Summary of Python Main Function Best Practices, Get a sample chapter from Python Tricks: The Book, Python Modules and Packages – An Introduction, How to Publish an Open-Source Python Package to PyPI, Python import: Advanced Techniques and Tips, What the best-practices are for what code to put into your, Running a computation that takes a long time, Printing information that would clutter the user’s terminal, Prints some output to tell the user that the data processing is starting, Pauses the execution for three seconds using, Prints some output to tell the user that the processing is finished, Reads a data file from a source that could be a database, a file on the disk, or a web API, Writes the processed data to another location, The name of the module, if the module is being imported. It is important that after defining the main function, you call the code by if__name__== "__main__" and then run the code, only then you will get the output "hello world!" Next, your script called process_data() and passed data in for modification. What if you want process_data() to execute when you run the script from the command line but not when the Python interpreter imports the file? The function name is followed by parameter(s) in (). If you are familiar with other programming languages like C++ and Java there you must have seen the main() function, that executes first before any other code statement when the code is compiled. Email, Watch Now This tutorial has a related video course created by the Real Python team. Here is how to call main() when the program is executed: if __name__ == '__main__': main() Save the program as "hello.py" (without the quotes). For python main function, we have to define a function and then use if __name__ == '__main__' condition to execute this function. Therefore, the best practice is to include most code inside a function or a class. if statement == True, and the script in _main_will be executed, if statement == false, and the script in __main__ will not be executed. However, there is a difference in the output from the third print(). 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. Here, we got two pieces of print- one is defined within the main function that is "Hello World" and the other is independent, which is "Guru99". Tweet 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(). 1. Then you can create a default workflow in main(), and you can have the best of both worlds. To demonstrate the results of importing your execution_methods.py file, start the interactive Python interpreter and then import your execution_methods.py file: In this code output, you can see that the Python interpreter executes the three calls to print(). Python interpreter, however, runs each line serially from the top of the file and has no explicit main() function.. Python offers other conventions to define the execution point. Main function is executed only when it is run as a Python program. In programming languages, when an operating system runs a program, a special function called main() is executed automatically. Now you should execute the execution_methods.py script from the command line, as shown below: In this example, you can see that __name__ has the value '__main__', where the quote symbols (') tell you that the value has the string type. … The first statement of a function can be an optional statement - the documentation string of the function or docstring. A Python method is like a Python function, but it must be called on an object. 当该python脚本被作为模块(module)引入(import)时，其中的main()函数 … By contrast, Python does not have a special function that serves as the entry point to a script. 3. Python main function. Join us and get access to hundreds of tutorials, hands-on video courses, and a community of expert Pythonistas: Real Python Comment Policy: The most useful comments are those written with the goal of learning from or helping out other readers—after reading the whole article and all the earlier comments. Some programming languages have a special function called main() which is the execution point for a program file. Finally, the value of modified_data is printed. The main() function appears at the bottom of a program, after all modules have been imported and functions have been declared. 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. Example. First, the data is created from read_data_from_web(). Python files are called modules and they are identified by the .py file extension. The import process caused the Python interpreter to execute all of the lines of code in the best_practices.py file, so the output shows the line explaining the purpose of the file. This conditional will evaluate to True when __name__ is equal to the string "__main__". A module can define functions, classes, and variables. There are two primary ways that you can instruct the Python interpreter to execute or use code: You can read a lot more about these approaches in How to Run Your Python Scripts. This is because when the Python interpreter encounters the def or class keywords, it only stores those definitions for later use and doesn’t actually execute them until you tell it to. This example uses repr() to emphasize that the value of __name__ is a string. Now the sys.exit() calls are annoying: when main() calls sys.exit(), your interactive Python interpreter will exit! The value of __name__ is: 'execution_methods', "This is my file to demonstrate best practices.". Following code also works. They are required to be part of a special function that is automatically invoked by the operating system when the program is executed. "This is my file to test Python's execution methods. 06:36 Although Python does not assign any significance to a function called main(), the best practice here is to name the entry point function main() anyways. Python interpreter uses the main function in two ways direct run: __name__=__main__ if statement == True, and the script in _main_will be executed import as a module __name__= module's filename if statement == false, and the script in __main__ will not be executed When the code is executed, it will check for the module name with "if." What’s your #1 takeaway or favorite thing you learned? When the code is executed, it will check for the module name with "if." ascii (object) ¶. Now that you can see the differences in how Python handles its different execution modes, it’s useful for you to know some best practices to use. This data is passed to process_data(), which returns the modified_data. How to Write a Python main Function? But, it can be quite confusing that the code at the bottom of a program is the main … In addition, main() should contain any code that you want to run when the Python interpreter executes the file. In this example, you have modified main() so that it calls the data reading, data processing, and data writing functions in turn. The result is a valid Python expression. When the if statement evaluates to True, the Python interpreter executes main(). Python is a very interesting programming language to learn. So when the interpreter runs a module, the __name__ variable will be set as __main__ if the module that is being run is the main program. By contrast, Python does not have a special function that serves as the entry point to a script. Now, what will happen when you execute this file as a script on the command line? Although in Python you can call the function at the bottom of your program and it will run (as we have done in the examples above), many programming languages (like C++ and Java) require a main function in order to execute. © 2012–2021 Real Python ⋅ Newsletter ⋅ Podcast ⋅ YouTube ⋅ Twitter ⋅ Facebook ⋅ Instagram ⋅ Python Tutorials ⋅ Search ⋅ Privacy Policy ⋅ Energy Policy ⋅ Advertise ⋅ Contact❤️ Happy Pythoning! Assuming the Python interpreter is in your execution path, you can type: python hello.py hello world ! Remember that, in Python, there is no difference between strings defined with single quotes (') and double quotes ("). 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(). Knowing the value of the __name__ variable is important to write code that serves the dual purpose of executable script and importable module. Assuming the Python interpreter is in your execution path, you can type: python hello.py hello world ! Enjoy free courses, on us →, by Bryan Weber It was designed to store and transport... What is Python Matrix? Complete this form and click the button below to gain instant access: "Python Tricks: The Book" – Free Sample Chapter (PDF). Watch it together with the written tutorial to deepen your understanding: Defining Main Functions in Python. Many programming languages have a special function that is automatically executed when an operating system starts to run a program. Practically, there isn’t much difference between them. Python also accepts function recursion, which means a defined function can call itself. In Python, defining the function works as follows. When the program is run, the python interpreter runs the code sequentially. Unsubscribe any time. You can call a Python function at the bottom of your program and it will run. A program gets executed only when it is executed from the main function and it does not get executed when it is imported as a module. Using this if, we can make Python behave like them, which feels more familiar for many people. It means that a function calls itself. Get a short & sweet Python Trick delivered to your inbox every couple of days. Defining the main function in Python is not mandatory but it is a good practice to do so for better readability of your program. len(sys.argv) is the number of command-line arguments. But if it’s directly part of the code then it will be executed when the file is … But in Python, the code executes line-by-line from top to bottom, so there is no specific main() function that should be called first.. You can use the len() to get the length of the given... What is XML? best-practices Then, you define a function called process_data() that does five things: Execute the Best Practices File on the Command Line. You will learn about four best practices to make sure that your code can serve a dual purpose: Remember that the Python interpreter executes all the code in a module when it imports the module. Convert an integer number to a binary string prefixed with “0b”. As Python is an interpreted language, it follows a top-down approach. You can read more about repr() in the Python documentation. It can also make our programs easier for non-Python programmers to read.W… Script: A Python script is a file that you intend to execute from the command line to accomplish a task. Nevertheless, having a defined starting point for the execution of a program is useful for understanding how a program works. Although Python does not assign any significance to a function named main(), the best practice is to name the entry point function main() anyways. The commands that you type will go after the >. 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 rest of the article, we will use the word "inner function" and "nested function" inter… Join us and get access to hundreds of tutorials, hands-on video courses, and a community of expert Pythonistas: Master Real-World Python SkillsWith Unlimited Access to Real Python. The main function in Python acts as a point of execution for any program. Use the different values of __name__ to determine the context and change the behavior of your code with a conditional statement. Next, you use print() to print a sentence describing the purpose of this code. When the Python interpreter imports code, the value of __name__ is set to be the same as the name of the module that is being imported.
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