Py4Inf-02-Expressions.pdf

Variables, Expressions, and Statements Chapter 2 Python for Informatics: Exploring Information Unless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution 3.0 License. /licenses/by/3.0/. Copyright 2010,2011, Charles R. Severance Constants • Fixed values such as numbers, letters, and strings are called “constants” - because their value does not change • Numeric constants are as you expect • String constants use single-quotes () print 123 or double-quotes () 123 print 98.6 98.6 print Hello world Hello world Variables • A variable is a named place in the memory where a programmer can store data and later retrieve the data using the variable “name” • Programmers get to choose the names of the variables • You can change the contents of a variable in a later statement x = 12.2 x 12.2 100 y = 14 x = 100 y 14 Python Variable Name Rules • Must start with a letter or underscore _ • Must consist of letters and numbers and underscores • Case Sensitive • Good: spam eggs spam23 _speed • Bad: 23spam #sign var.12 • Different: spam Spam SPAM Reserved Words • You can not use reserved words as variable names / identifiers and del for is raise assert elif from lambda return break else global not try