What Is the Python Standard Library, and How Do You Import a Module?

In software development, a library is like a toolbox for developers. Instead of having to implement every single part of the code yourself from scratch, a library gives you pre-written and reusable code, like functions, classes, and data structures that you can use in your projects. Python has an extensive standard library with many different built-in modules. They're all standardized, well-vetted solutions for many of the problems and tasks you'll face daily as a programmer, such as: * Interacting with the operating system. * Working with files. * Networking. * Working with date and time. * Performing mathematical operations. * Using regular expressions. * Testing and debugging your code. * And much more! Some examples of popular built-in modules are `math`, `random`, `re` (short for "regular expressions"), and `datetime`. The `math` module has helpful functions for performing more complex mathematical operations. The `random` module is helpful for generating random numbers. The `re` module is used for working with regular expressions. And the `datetime` module is helpful for working with dates and times in Python. But how can you access the variables, constants, functions, and classes defined in these built-in modules? You use an import statement. These statements let you import modules into your Python script. Import statements are generally written at the top of the file. Also, you can customize them based on your needs. First, you use the `import` statement, followed by the name of the module: ```python import module_name ``` Let's say that you want to import the `math` module. In that case, you would write this at the top of your file: ```python import math ``` Then, if you need to call a function from that module in your Python script, you would use dot notation, with the name of the module followed by the name of the function: ```python module_name.function_name() ``` For example, to get the square root of 36, you would write `math` followed by a dot and then `sqrt`, an abbreviation of square root, and within parentheses, you would pass any necessary arguments. In this case, we only need to pass in the number we want the square root of: ```python math.sqrt(36) ``` This is the most basic version of an import statement, but there are other alternatives. If you need to import the module with a different name (also known as an "alias"), you can use this syntax, with `as` followed by the alias at the end of the import statement: ```python import module_name as module_alias ``` This is often used to shorten long module names, or to avoid naming conflicts. For example, to refer to the `math` module as `m` in your code, you can assign an alias to it, like this: ```python import math as m ``` Then, you can access the elements of the module using the alias: ```python m.sqrt(36) ``` But sometimes you don't need to import everything from a module. Perhaps you only need one or two specific functions or classes. Python has exactly what you need in that case. Now the import statement starts with `from`, followed by the name of the module, and then the `import` keyword followed by the name of the elements that you want to import: ```python from module_name import name1, name2 ``` Then, you can use these names without the module prefix in your Python script. If you want to assign aliases to these names, you can do that by using the `as` keyword after each, followed by the alias you want to use: ```python from module_name import name1 as alias1, name2 as alias2 ``` Let's say that you only want to import the radians, sine, and cosine functions from the `math` module. You would write: ```python from math import radians, sin, cos ``` Now you can call these functions directly in your code, without the `math` module as a prefix. Here we have a more detailed example: To find the sine and cosine of a specific angle initially expressed in degrees, we can call the `radians` function to convert it to radians, and then call the sine and cosine functions, passing the angle in radians: ```python from math import radians, sin, cos angle_degrees = 40 angle_radians = radians(angle_degrees) sine_value = sin(angle_radians) cos_value = cos(angle_radians) print(sine_value) # 0.6427876096865393 print(cos_value) # 0.766044443118978 ``` Notice how we are calling the functions directly, without the name of the module as a prefix. This is because we imported the functions with this alternative syntax. This is helpful, but it can result in naming conflicts if you already have functions or variables with the same name defined in the Python script itself. So that's something to keep in mind when choosing which type of import statement you want to use. And finally, we find this import statement that ends with an asterisk. The asterisk is telling Python that you want to import everything in that module, but you want to import it so that you don't need to use the name of the module as a prefix: ```python from module_name import * ``` For example, if you do this while importing the `math` module, you'll be able to call any function defined in that module without specifying the name of the module as a prefix. Here are some examples: ```python from math import * print(sqrt(36)) # 6.0 print(pow(5, 2)) # 25.0 print(exp(1)) # 2.718281828459045 ``` However, this is generally discouraged because it can lead to namespace collisions, and make it harder to know where certain names are coming from. Import statements work exactly the same for functions, classes, constants, variables, and any other elements defined in the module. Here is an example of a constant from the `math` module, the number `pi`: ```python import math print(math.pi) ``` And here is an example of a class from the `datetime` module. We create a date object that represents July 15, 1959. Then, we assign that `date` object to a variable and access the day, month, and year individually using dot notation: ```python import datetime birthday = datetime.date(1959, 7, 15) print(birthday.day) # 15 print(birthday.month) # 7 print(birthday.year) # 1959 ``` You can find more information about the content of the module in the official Python documentation for that module. Great. Now that you know more about modules, you should also know about this very important idiom in Python scripts, because they are very closely related: ```python if __name__ == '__main__': # Code ``` `__name__` is a special built-in variable in Python. When a Python file is executed directly, Python sets the value of this variable to the string `"__main__"`. But if the Python file is imported as a module into another Python script, the value of the `__name__` variable is set to the name of that module (usually the filename without the `.py` extension). This is why you'll often find this conditional in Python scripts. It contains the code that you want to run **only if** the Python script is running as the main program: ```python if __name__ == '__main__': # Code ``` But if the script is imported as a module, the code within that block doesn't run. This is helpful because it allows Python scripts to have two purposes. They can be run directly to execute their main logic, or they can be imported into another module without executing their main logic.