Day 1 Lesson 3 Variables Expressions and Statements

Day 1 – Lesson 3 Variables, Expressions, and Statements Python Mini-Course University of Oklahoma Department of Psychology 1 Python Mini-Course: Day 1 - Lesson 3 4/5/09

Lesson objectives 1. Identify the data types and keywords in the Python language 2. Understand how variable assignment works in Python 3. Identify and use math, string, and comparison operators 4. Write in-code comments and docstrings 2 Python Mini-Course: Day 1 - Lesson 3 4/5/09

Data types Python has multiple data types Built-in User defined Numeric types: int, float, long, complex string Boolean 3 Python Mini-Course: Day 1 - Lesson 3 4/5/09

Data types Use the type() function to find the type for a value or variable Data can be converted using cast commands Ex. a = str(17) type(a) 4 Python Mini-Course: Day 1 - Lesson 3 4/5/09

Variables As in mathematics, much of the power of computer programs is in the use of variables Python variables are identified by name 5 Python Mini-Course: Day 1 - Lesson 3 4/5/09

Variable naming conventions Must start with a letter Can contain letters, numbers, and underscore character (_) Can be any (reasonable) length Case sensitive big is NOT the same as BIG 6 Python Mini-Course: Day 1 - Lesson 3 4/5/09

Naming style guidelines Use descriptive names Use lowercase with underscores for variable names Don’t use “I”, “l”, or “O” for single letter names Be consistent Except when it makes code easier to read 7 Python Mini-Course: Day 1 - Lesson 3 4/5/09

Keywords You cannot use any of the keywords for variable names and assert break class continue def 8 del elif else except exec finally Python Mini-Course: Day 1 - Lesson 3 for from global if import in is lambda not or pass print raise return try while yield 4/5/09

Assignment and initialization Variables can be created at any time (no need to declare them) Variable names refer to info in memory The Python interpreter keeps track of the namespace Two variables can refer to the same memory location 9 Python Mini-Course: Day 1 - Lesson 3 4/5/09

Math operators Arithmetic +, -, *, and / Note: the / operator performs floor division on int data types Try: minute = 59 minute/60 Exponentiation ** Order of operations (PEMDAS) 10 Python Mini-Course: Day 1 - Lesson 3 4/5/09

String operators String concatenation Use + sign print ‘this’ + ‘is a string’ String repetition Use * sign lunch = ‘Spam’ * 4 print lunch 11 Python Mini-Course: Day 1 - Lesson 3 4/5/09

Comparison operators Perform logical comparison and return Boolean value Equality == Inequality<> or != Less than < Greater than > (or equal to) <= >= is 12 Python Mini-Course: Day 1 - Lesson 3 4/5/09

Comments The # symbol indicates the rest of the line is a comment Good programming practice: Use comments liberally 13 Python Mini-Course: Day 1 - Lesson 3 4/5/09

docstrings The built-in Py. Doc module can automatically generate documentation if the programmer uses docstrings Ex: doctest. py 14 Python Mini-Course: Day 1 - Lesson 3 4/5/09