Python Basics (2021-11-17 - 2021-11-19)

Day 1

Day 2

  • while Loops

  • for Loops

    • while and for in C (similar in Jave and C#): for is just a shorthand for while - as opposed to Python.

      code/loops.c
      #include <stdio.h>


int main(void)
{
    int i = 0;
    int s = 0;
    while (i <= 10) {
        s += i;
        i += 1;
    }
    printf("sum (while): %d\n", s);

    s = 0;
    for (i = 0; i <= 10; i++)
        s += i;
    printf("sum (for): %d\n", s);
    
    return 0;
}

  • Sequential Datatypes

  • Exercise: Function: uniq()

    ../../../../_images/uniq-algo.png
    code/uniq.py
    lin = [2,1,3,10,3,3,1,10,5,2]

lout = []
have = set()  # maintained in parallel to answer the "schon gesehen?"
              # question faster

for element in lin:
    if element not in have:
        lout.append(element)
        have.add(element)

print('outputliste:', lout)
print('have:', have)

  • File I/O

  • Exercise: Parsing /etc/passwd

    • Solution 1: relatively explicit and long

      code/getuser.py
      import sys
import pprint


filename = sys.argv[1]
username = sys.argv[2]

userdatabase = {}

f = open(filename)
for line in f:
    line = line.rstrip()   # remove linefeed at end of line
    fields = line.split(':')

    username = fields[0]
    password = fields[1]
    userid = int(fields[2])
    groupid = int(fields[3])
    description = fields[4]
    home = fields[5]
    loginshell = fields[6]

    userrecord = {}
    userrecord['username'] = username
    userrecord['password'] = password
    userrecord['userid'] = userid
    userrecord['groupid'] = groupid
    userrecord['description'] = description
    userrecord['home'] = home
    userrecord['loginshell'] = loginshell

    # pprint.pprint(userrecord, indent=4)

    userdatabase[username] = userrecord
    
#pprint.pprint(userdatabase, indent=4)

pprint.pprint(userdatabase[username])

    • Solution 2: dense, short, requires a bit of programming pratice (not much though)

      code/getuser-dense.py
      import sys
import pprint


filename = sys.argv[1]
username = sys.argv[2]

userdatabase = {}

f = open(filename)
for line in f:
    username, password, userid, groupid, description, home, loginshell = line.rstrip().split(':')
    userdatabase[username] = {
        'username': username,
        'password': password,
        'userid': userid,
        'groupid': groupid,
        'description': description,
        'home': home,
        'loginshell': loginshell,
    }

pprint.pprint(userdatabase[username])

Day 3

  • Back to Exercise: Function: uniq(): set usage for speed!

  • The range Function

  • Iteration, Comprehensions, and Generators

  • Fibonacci Numbers: 1, 1, 2, 3, 5, 8, 13, …

    code/fibonacci.py
    def fibo():
    '''
    Generate the *infinite* fibonacci sequence. Note that we use the
    ``yield`` statement and not ``return`` - this makes this a
    *generator* which is fundamentally different from a *function*.
    '''

    first = second = 1

    yield first
    yield second

    while True:
        next = first + second
        yield next
        first, second = second, next

if __name__ == '__main__':    # started as program, and not imported
    nums = fibo()
    for i in nums:
        print(i)

  • Iterable and dict, list. See notebook.

  • Comprehensions. See notebook.

  • More About Lists

  • More About Dictionaries

  • More About Sets

  • References, (Im)mutability

  • Functions

  • Modules and Packages

  • CSV Files

  • Exercise: Read CSV File

    The module stock.py:

    code/csv/stock.py
    import csv
import datetime

# note: functions prefixed with '_' are not imported when someone says
# "from there import *". those functions are internal to this module,
# so it is good practice to hide them in such a way.

def _fix_float_number(record, fieldname):
    old = record[fieldname]
    old = old.replace(',', '.')
    new = float(old)
    record[fieldname] = new

def _fix_bool(record, fieldname):
    old = record[fieldname]
    if old == 'T':
        new = True
    elif old == 'F':
        new = False
    else:
        new = True
    record[fieldname] = new

def _fix_date(record, fieldname):
    old = record[fieldname]
    new = datetime.datetime.strptime(old, '%Y%m%d')
    record[fieldname] = new

def read_stock(filename):
    csvfile = open(filename)
    rdr = csv.DictReader(csvfile, delimiter=';')

    converted_records = []
    for record in rdr:
        _fix_date(record, 'AS_OF_DATE')
        _fix_bool(record, 'FUND_IN_UNIVERSE')
        _fix_bool(record, 'FUND_OF_FUNDS')
        _fix_float_number(record, 'FUND_HOLDINGS_COUNT')
        _fix_float_number(record, 'FUND_HOLDING_FUNDS_COUNT')
        converted_records.append(record)

    return converted_records

    The program read-stock.py:

    code/csv/read-stock.py
    import stock
import sys
from pprint import pprint

for record in stock.read_stock(sys.argv[1]):
    pprint(record)

  • Databases

  • Exercise: Read CSV File, Write into Sqlite3 Database

    The program stock-to-db.py:

    code/csv/stock-to-db.py
    import stock

import sys
import sqlite3

csvname = sys.argv[1]
dbname = sys.argv[2]

db = sqlite3.connect(dbname)

for record in stock.read_stock(csvname):
    stmt = f'''
    insert into records values (
        "{record['FUND_ISIN']}", 
        "{record['FUND_NAME']}", 
        "{record['FUND_TICKER']}"
    );
    '''
    db.execute(stmt)

db.commit()

Jupyter Notebook

PCAP Exam Syllabus

Exam block #1: Modules and Packages (12%)

Objectives covered by the block (6 items)

  • import variants; advanced qualifiying for nested modules

    Comment: os.path

  • dir(); sys.path variable

  • math: ceil(), floor(), trunc(), factorial(), hypot(), sqrt(); random: random(), seed(), choice(), sample()

  • platform: platform(), machine(), processor(), system(), version(), python_implementation(), python_version_tuple()

  • idea, __pycache__, __name__, public variables, __init__.py

  • searching for modules/packages; nested packages vs directory tree

Exam block #2: Exceptions (14%)

Objectives covered by the block (5 items)

  • except, except:-except; except:-else:, except (e1,e2)

  • the hierarchy of exceptions

  • raise, raise ex, assert

  • event classes, except E as e, arg property

  • self-defined exceptions, defining and using

Exam block #3: Strings (18%)

Objectives covered by the block (8 items)

  • ASCII, UNICODE, UTF-8, codepoints, escape sequences

  • ord(), chr(), literals

  • indexing, slicing, immutability

  • iterating through,

  • concatenating, multiplying, comparing (against strings and numbers)

  • in, not in

  • .isxxx(), .join(), .split()

  • .sort(), sorted(), .index(), .find(), .rfind()

Exam block #4: Object-Oriented Programming (34%)

Objectives covered by the block (12 items)

  • ideas: class, object, property, method, encapsulation, inheritance, grammar vs class, superclass, subclass

  • instance vs class variables: declaring, initializing

  • __dict__ property (objects vs classes)

  • private components (instance vs classes), name mangling

  • methods: declaring, using, self parameter

  • instrospection: hasattr() (objects vs classes), __name__, __module__, __bases__ properties

  • inheritance: single, multiple, isinstance(), overriding, not is and is operators

  • constructors: declaring and invoking

  • polymorphism

  • __name__, __module__, __bases__ properties, __str__() method

  • multiple inheritance, diamonds

Exam block #5: Miscellaneous (List Comprehensions, Lambdas, Closures, and I/O Operations) (22%)

Objectives covered by the block (9 items)

  • list comprehension: if operator, using list comprehensions

  • lambdas: defining and using lambdas, self-defined functions taking lambda as as arguments; map(), filter();

  • closures: meaning, defining, and using closures

  • I/O Operations: I/O modes, predefined streams, handles; text/binary modes

  • open(), errno and its values; close()

  • .read(), .write(), .readline(); readlines() (along with bytearray())