Introduction to Python

by K. Yue

1. Resources

• Python: https://www.python.org/
• Version 3.5:
• Manual: http://docs.python.org/3.5/reference/index.html
• Tutorial: https://docs.python.org/3.5/tutorial/interpreter.html
• Standard library: https://docs.python.org/3.5/library/index.html
• Python's Beginner Guide for programmers: https://wiki.python.org/moin/BeginnersGuide/Programmers
• Style Guide for Python Code: https://www.python.org/dev/peps/pep-0008/
• You are recommended to install Python using customization to put Python in the top level of your C drive. For example: in c:\Python35-32\.

2. Basics

• Python is a (very) high-level, open source, general-purpose, object-oriented, extensible, interpreted scripting programming language.
• Python is a scripting language:
  1. Especially good for scripting and rapid application development
  2. Object-oriented language (more so than many other OO languages)
  3. Dynamic typing: an object bound to a variable can change type dynamically.
  4. Strongly typed: less implicit type conversion.
  5. Native high level data structures: list, tuple, set, dictionary, etc.
  6. Interpreted
  7. Automatic garbage collection
  8. Open source
  9. Use good software engineering principles

Python programs

• A Python program contains logical lines.
• A logical line usually contain one physical line, but can be extended by '\' to contain many physical lines.
• Implicit line joining is performed for (), [] and {} expressions. It is preferred to using '\'.
• Do not use ';' as a statement separator.
• Indentation (white spaces at the beginning of lines) has meaning. They can be mandatory.
• Indentation level is used to group statements.
• Python 2: do not mix tabs and spaces.
• Python 3:
  • Disallow mixing of tabs and spaces.
  • Spaces are preferred.
• Comments start with a '#'.

Running Python in Windows

• To start the Python interpreter, open a command line prompt and execute:

set path=%path%;C:\Python35-32
python

• You may need to replace "C:\Python35-32" by the location of your python installation.
• Use Control-z or quit() to exit Python interpreter.
• To run a Python program, helloworld.python, use:

python helloworld.py

How to learn a new language:

1. syntax
2. concepts that are familiar to you: notice any difference.
3. concepts that are new to you
4. resources and libraries
5. design patterns and best practices

Some basics to get you interested (hopefully):

• No ;
• No i++
• Multiple assignment: a,b = b,a
• strings are automatically concatenated: print("ab" "cd")
• strings are automatically indexed:
  • a='12345'
  • print(a[1:3])
• id() to find out id of an object.
• indentation is mandatory, not optional.
• arrays are much more general.

Example of concepts that may be new to you:

• Immutable or mutable objects?
  • The value of an immutable object cannot be changed.
  • Use the id() function for experimentation
• Automatic garbage collection: "Objects are never explicitly destroyed; however, when they become unreachable they may be garbage-collected."

Example:

Interpreter:

C:\Users\yue>python
Python 3.5.1 (v3.5.1:37a07cee5969, Dec  6 2015, 01:38:48) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> hello_world = 'hello world, from ITEC 3335'
>>> print(hello_world)
hello world, from ITEC 3335
>>> exit()

Print first 100 Fibonacci numbers:

#   print first 100 fibonacci numbers
a, b, count = 0, 1, 1
while count <= 100:
   print(b, ' ', end='')
   a, b, count = b, a+b, count+1
print()     

Note the use of multiple assignment statements. In other languages, such as Java, you may need to replace:

a, b, count = b, a+b, count+1

by

temp=b;
b=a+b;
a=temp;
count++;

Other examples as demand in the class.

Example: A Non-trivial program (if needed)

Consider the following weather information file: 201401daily_sample.csv. Write a Python program to read the 'codeSum' column (#23) and output the count of each codeSum. Note that the codeSum columns may contain more than one codeSum separated by white spaces. Example:

SN FG+ FZFG BR UP

has five codeSum:

• SN
• FG+
• FZFG
• BR
• UP

codeSum.py: more Java-like style

import sys, getopt
import re
from operator import itemgetter

#   getopt: C-style parser for command line options.
#   sys: System-specific parameters and functions.
#   re: regular expression operation
#   operator: methods for built-in operators.
#       (useful when passing the function as a parameter.)

#   Read and process a weather information file.
#   It parse the codeSum column (#23) in the CSV file
#   and show the count of each codeSum.

f = open(sys.argv[1], 'r')
result = [];
heading = f.readline().split(',')
num_line = 0;

for line in f:
    line = line.rstrip()    #   strip trailing white spaces
    #   result is an array of arrays:
    #   Add the array contains columns of the current line
    #   to result.
    result.append(line.split(','))
    num_line = num_line + 1
f.close()

#   Debug:
#   for i in range(len(result)):
#       print (str(i) + ":" + str(result[i][22]))

#   count is a dictionary with the key being the individual CodeSummary
count = {};
for i in range(len(result)):
    #   process one reading.
    line = result[i][22].strip()
    if line:
        #   Get all codeSummary and update their counts.
        summary = re.split('\s+', line)
        for j in range(len(summary)):
            #   Debug:
            #   print (str(j) + ":" + str(summary[j]) + "---")
            if summary[j] in count.keys():
                count[summary[j]] += 1
            else:
                count[summary[j]] = 1

#   Print result in the sorted order of codeSum.               
for key, value in sorted(count.items(), key=itemgetter(0)):
    print (key + ": " + str(value))

           
Running the program:

>...python codeSum.py 201401daily_sample.csv
BR: 18
FG: 2
FG+: 5
FZFG: 8
HZ: 10
RA: 2
SN: 14
UP: 4

If you are really Python curious, a second, more Python-like version:

from collections import defaultdict
import sys, getopt
import re
from operator import itemgetter

#   Read and process a weather information file.
#   It parse the codeSum column (#23) in the CSV file
#   and show the count of each codeSum.

f = open(sys.argv[1], 'r')
heading = f.readline().split(',')
result = [line.strip().split(',') for line in f.readlines()]
f.close()

#   count is a dictionary with the key being the individual CodeSummary
count = defaultdict(int)
for field in filter(lambda a: a, map(lambda w: w[22].strip(), result)):
   for sym in re.split('\s+', field):
      count[sym] += 1
  
#   Print result in the sorted order of codeSum.               
for key, value in sorted(count.items(), key=itemgetter(0)):
    print (key + ": " + str(value))