Lecture 2

Python tour part I

Types, control structures, I/O

MCS 275 Spring 2022
David Dumas

Lecture 2: Python tour

• Read the syllabus
• Discord open (link in the zoom chat or Blackboard).
• Homework 1 schedule adjustment due to MLK holiday: deadline will be Noon on Wed 19 January.

Plan for Today

Start our quick tour of Python, summarizing some material I think you saw in a previous course^*.

I'll indicate where you can find more detail in optional texts and the online MCS 260 materials from my Fall 2021 course.

* If I mention things today that are completely new to you, please let me know afterward.

Textbook note

The most comprehensive optional text is

Written in 2013, so it discusses Python 3 and Python 2. Since then, Python 2 has been phased out. We only talk about Python 3.

Notes for self study

I'll do most examples as live coding today.

Options to study this outside of lecture:

• These slides: Main points summarized succinctly.
• MCS 275 Python tour: Lots of code examples.
• All the MCS 260 lecture slides: Much more detailed (perhaps too detailed)

Scripts and REPL

Two ways to run Python code:

• One statement at a time, in interactive mode, also known as the REPL (read-eval-print loop)
• A whole file at a time, in script mode

See Lutz, Chapter 3 or MCS 260 Lec 2.

Variables and types

Create new vars by assignment, name = value

Dynamically typed: No need to specify the type of a variable, nor for it to remain the same.

Basic types include: int, float, boolean, string, None

See Lutz, Chapters 4-6 and MCS 260 Lec 3.

Lists and dicts

Lists are mutable ordered collections of elements, accessible by integer index.

[260,275,"hello",True,None,None,-1.5]

Dictionaries (dicts) are mutable key-value mappings. Index like lists, but use key instead of position.

{ "name": "Stinger", "age": 403, 
  "species": "space wasp", "hostile": True }

See Lutz, Chapter 8 and MCS 260 Lec 5 and Lec 10.

Strings

Strings support some list-like features, such as indexing and slicing.

Lists have useful methods such as .lower(), .startswith(...), .format(...), and many more.

See Lutz, Chapter 7 and MCS 260 Lec 7.

if-else-elif

If statement (or conditional) runs a block of code only if a condition is True. Elif/else allow chained tests.

        if GREAT:
            RUNS_IF_GREAT_IS_TRUE
        elif OKAY:
            RUNS_IF_OKAY_IS_TRUE_AND_GREAT_IS_FALSE
        else:
            RUNS_OTHERWISE
        

Non-boolean conditions are coerced: empty list, empty dict, empty string, None, and zero map to False.

See Lutz, Chapter 12 and MCS 260 Lec 6 and Lec 18.

Loops

While: Keep going until a condition becomes False

            while CONDITION:
                STUFF_TO_DO  # should modify things in the condition
        

For: Take items (list elements, dict keys) out, one at a time, and do something with each.

            for ITEM in CONTAINER:
                STUFF_TO_DO  # should use the ITEM
        

See Lutz, Chapter 13 and MCS 260 Lec 6.

Files

open(filename,mode,...) opens a file and returns a file object. Mode string selects reading ("r"), writing ("w"), ...

Methods of the file object perform input/output (I/O).

Read/write text to text files ("t" in mode), bytes to binary files ("b" in mode).

.close() a file when finished.

The basics are in Lutz, Chapter 9 and MCS 260 Lec 13 and Lec 14.