Chemistry 311

Functions from Libraries

The functions discussed in the previous module are built-in functions. They are always available because they are part of the Python language itself.

Python also provides many additional functions that are grouped into libraries (also called modules). Each library contains functions that are designed for a particular purpose. For example, the math library contains functions for performing common mathematical calculations.

Before you can use a function from a library, you must first import the library.

  import math
  

Once a library has been imported, its functions are accessed using dot notation:

  math.function_name(arguments)
  

The name before the period identifies the library, while the name after the period identifies the function you wish to use.

The math.sqrt() function

The sqrt() function calculates the square root of a number.

  import math

  x = math.sqrt(25)

  print(x)
  

Output

  5.0
  

The math.log10() function

The log10() function calculates the common (base-10) logarithm of a number. This function is especially useful in chemistry because quantities such as pH and pOH are defined using base-10 logarithms.

  import math

  x = math.log10(1000)

  print(x)
  

Output

  3.0
  

Python contains many libraries besides math. Throughout this course you will use additional libraries to create graphs, process data, communicate with laboratory equipment, and perform scientific calculations. The process is always the same: import the library, then call its functions using dot notation.

Big idea: Built-in functions are always available, while library functions must first be imported. Libraries allow Python to perform tasks far beyond the capabilities of the built-in functions alone.

Finding information about library functions

No programmer memorizes every function in every library. Fortunately, Python provides several ways to learn about a library and the functions it contains.

Listing the contents of a library

The built-in dir() function lists the names of the objects contained in a library.

  import math

  print(dir(math))
  

This displays a long list containing the names of all the functions and constants available in the math library, such as sqrt, log10, sin, cos, and pi.

Getting help on a function

The built-in help() function displays documentation about a library or one of its functions.

  import math

  help(math.sqrt)
  

Python displays information describing what the function does, the arguments it expects, and the value that it returns.

You can also ask for help on an entire library:

  help(math)
  

This displays documentation for all of the functions and constants in the math library.

Tip: In VS Code, hovering your mouse over a function name or typing an opening parenthesis after the function name often displays the function's documentation and expected arguments without leaving the editor.

Finding information about a library

Python includes hundreds of libraries containing thousands of useful functions. No programmer memorizes all of them! Instead, programmers learn how to explore a library and read its documentation.

Listing the contents of a library

The built-in dir() function displays the names of the objects contained in a library.

  import math

  print(dir(math))
    

This command displays a long list containing the names of the functions and constants available in the math library. For example, you will see names such as sqrt, log10, sin, cos, pi, and e.

Getting help on a function

The built-in help() function displays documentation for a library or one of its functions.

  import math

  help(math.sqrt)
    

Python displays information describing what the function does, the arguments it expects, and the value that it returns.

You can also request help for an entire library.

  help(math)
    

Using library constants

Libraries often contain useful constants in addition to functions. Constants are accessed using the same dot notation.

  import math

  print(math.pi)
  print(math.e)
    

Output

  3.141592653589793
  2.718281828459045
    

Documentation in Visual Studio Code

When using Visual Studio Code, you often do not need to leave the editor to learn about a function. As you type a function call, or when you hover your mouse over the function name, VS Code displays information about the function, including its purpose and the arguments it expects.

For example, typing

  math.log10(
    

will display a tooltip showing the function's expected argument and a brief description of what it does.

Remember: Experienced programmers do not memorize every library function. They know how to explore a library, read its documentation, and find the information they need.

Worked examples

    from math import log10 as log
    

    from math import log10 as log

    H_conc = 0.0135

    pH = -log(H_conc)

    print(f"The pH of the solution is {pH:.3f}.")
    

    The pH of the solution is 1.870.
    

    from math import log10 as log
    pH = -log(H_conc)
    

    import math
    pH = -math.log10(H_conc)
    

Practice

Key points (one glance)

from math import log10 as log

from math import log10 as log

Key points (one glance)

• Python libraries (also called modules) contain additional functions that extend the capabilities of the language.
• Before using a library function, you must import the library or the specific function you want to use.
• An entire library can be imported with import math.
• A single function can be imported with from math import log10.
• A function may be given a shorter name (an alias) using the as keyword: from math import log10 as log.
• Library functions require parentheses containing their arguments, for example: log(H_conc).
• The built-in dir() function lists the contents of a library, while help() displays documentation for a library or one of its functions.
• Libraries often contain useful constants as well as functions. For example, the math library provides math.pi and math.e.
• In Visual Studio Code, hovering over a function or typing an opening parenthesis often displays helpful documentation automatically.

Big picture: Python's built-in functions are only the beginning. By importing libraries, you gain access to thousands of specialized functions for mathematics, data analysis, graphing, scientific computing, and controlling laboratory equipment.