Lecture 2

Python tour

Types, control structures, I/O

MCS 275 Spring 2024
Emily Dumas

Lecture 2: Python tour

Reminders and announcements:

• Read the syllabus.
• Discord open (invite link on Blackboard).
• Homework 1 will be due Noon on Wed Jan 17. Not posted yet.

Notes for self study

These slides may be useful for self-study. I may not show them all today.

Also useful: MCS 260 materials and Lutz's book

String features

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

See Lutz, Chapter 7 and MCS 260 Lec 7.

Built-in data structures

• list - ordered collection of items, access by 0-based index
• dict - key-value mapping. Given a key, it returns the corresponding value.
• tuple- like a list, but immutable
• set - like a dict but there are no values.

Example list

[260,275,"hello",True,275,None,-0.5]

Example dict

{ "name": "Zorlax", "age": 1645, 
  "species": "Tau Cetian robot", "hostile": True }

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

Negative indices and slices

Negative indices get elements based on position relative to the end. Index -1 means the last element.

Slices get a contiguous sublist. L[a:b] gives a list consisting of all L[k] such that   a ≤ k < b.

Slices can also get evenly spaces items with L[a:b:stepsize]

Lists are mutable

Can be changed by item assignment (L[i] = newval) and various method calls (e.g. .sort, .append, .pop).

Mutations (statements that change list contents) often return nothing.

Strings can be indexed

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

But unlike lists, strings are immutable.

Membership in dict

If D is a dict, the boolean expression

        x in D
    

checks whether x is a key.

Control structures

Conditionals (if)

Loops (for, while)

(We won't discuss match, which is new in 3.10)

if-elif-else

Check a series of boolean expressions in order. As soon as one is true, run the code block below it.

        if CHECKED_FIRST:
            DO_THING_A
        elif CHECKED_SECOND:  # elif means "else, if"
            DO_THING_B
        elif CHECKED_THIRD:
            DO_THING_C
        else:
            DO_THING_D
        # Now exactly one of A,B,C,D has happened
        

Coercion

Non-boolean conditions are coerced if they appear where a boolean is expected.

Empty containers (list, dict), empty string, None, and zero become False, any other value becomes True.

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

Loops

While: Keep going until a condition becomes False

            while CONDITION:
                DO_THING  # maybe makes CONDITION true next time
        

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

            for ITEM_VARNAME in CONTAINER:
                DO_THING # should use ITEM_VARNAME
        

See Lutz, Chapter 13 and MCS 260 Lec 6.

Early exit or resume

break - jump out of the nearest enclosing loop immediately

continue - jump back to the nearest enclosing loop immediately

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.

Functions

def contains_all_digits(x):
    """Does integer x uses all decimal digits?"""
    for c in "0123456789":
        if c not in str(x):
            return False
    return True

>>> contains_all_digits(2*5)
False
>>> contains_all_digits(2**94)
True

See Lutz, Chapters 16-18 or MCS 260 Lec 9 and Lec 24.

Docstrings

The first non-comment statement in any Python file, function, class, or method should be a string literal on a line by itself.

Python remembers that string as a description of the file/function/class/method. Called a docstring.

Anywhere else you want to put explanatory text, use a comment.