GEOG 485:
GIS Programming and Software Development

1.5.5 Python syntax


Every programming language has rules about capitalization, white space, how to set apart lines of code and procedures, and so on. Here are some basic syntax rules to remember for Python:

  • Python is case-sensitive, both in variable names and reserved words. That means it’s important whether you use upper or lower-case. The all lower-case "print" is a reserved word in Python that will print a value, while "Print" is unrecognized by Python and will return an error. Likewise, arcpy is very sensitive about case and will return an error if you try to run a tool without capitalizing the tool name.
  • You end a Python statement by pressing Enter and literally beginning a new line. (In some other languages, a special character, such as a semicolon, denotes the end of a statement.) It’s okay to add empty lines to divide your code into logical sections.
  • If you have a long statement that you want to display on multiple lines for readability, you need to use a line continuation character, which in Python is a backslash (\).
  • Indentation is required in Python to logically group together certain lines, or blocks, of code. You should indent your code four spaces inside loops, if/then statements, and try/except statements. In most programming languages, developers are encouraged to use indentation to logically group together blocks of code; however, in Python, indentation of these language constructs is not only encouraged, but required. Though this requirement may seem burdensome, it does result in greater readability.
  • You can add a comment to your code by beginning the line with a # character. Comments are lines that you include to explain what the code is doing. Comments are ignored by Python when it runs the script, so you can add them at any place in your code without worrying about their effect. Comments help others who may have to work with your code in the future, and they may even help you remember what the code does.  The intended audience for comments is developers.
  • You can add documentation to your code by surrounding a block of text with """ (3 consecutive double quotes) on either side).  As with the # character, text surrounded by """ is ignored when Python runs the script.  Unlike comments, which are sprinkled throughout scripts, documentation strings are generally found at the beginning of scripts.  Their intended audience is end users of the script (non-developers).