Python Introduction Workshop

Workshop

Please keep in mind that Python cares about whitespace (spaces, returns, etc...), if you just paste the code in, be aware of what spaces you are also pasting in... ;-)

Create a file

$ nano myprog.py

Add the following line

print 'Hello World!'

Save the file and exit. Ctrl+O, Ctrl+X

Run the program.

$ python myprog.py

Add a comment to the top of your program. Comments are to help people read your program.

# Program by [username]

Create a variable to store input into the program.

# Import adds a library (addition functionality) to your program
# sys is the system library
import sys

# Read from stdin into a variable called instr
instr = sys.stdin.readline()

# Print message w/ variable
print 'Hello ' + instr + '.'

Run the program, this time with input.

$ whoami | python myprog.py

Use the input to alter the output, before your print message add:

if instr == 'jbg':
  instr = 'Programmer'
else:
  instr = 'Writer'

Save and run.

It doesn't work! This is because there is actually a return character in the string. Change the following:

instr = sys.stdin.readline().strip()

Save and run.

$ whoami | python myprog.py
$ echo 'Ray Bradbury' | python myprog.py

Create a new program (myprog2.py) which loops through all the lines coming from stdin.

 import sys

 for line in sys.stdin:
   print line.strip()

Run it.

$ cat /pub/451.txt | python myprog2.py

That's a lot of lines, how many exactly?

$ cat /pub/451.txt | python myprog2.py | wc -l

Simplest loop.

while True:
  print "hello."

Simple loop.

for i in range(0, 10):
  print "hello"

Let's store those lines in an array.

import sys

lines = []
for line in sys.stdin:
  lines.append(line.strip())

print 'Stored ' + str(len(lines)) + ' lines.'

451 remixed via a function. Add the following to the top of your program.

def remix(lines):
  lines = reversed(lines)
  for line in lines:
    print line[::-1] # This is extended slice syntax. It works by doing [begin:end:step]

Add the following to the end of your program.

remix(lines)

Run it.

$ cat /pub/451.txt | python myprog2.py | more

Try other splits.

print line[0:10]
print line[10:15] + line[0:10]

A more powerful remix.

$ python /pub/shaney.py /pub/451.txt

Scrape the internet into a file.

$ python /pub/scraper.py > internet.txt

Eat the internet w/ eat.py

import sys

def eat(lines):
  new_lines = []
  for line in lines:
    if len(line) > 0:
      new_line = line[0:len(line) - 1]
      print new_line
      new_lines.append(new_line)
  return new_lines

lines = []
for line in sys.stdin:
  lines.append(line.strip())

while len(lines) > 0:
  lines = eat(lines)

Run it.

$ cat internet.txt | python eat.py

Why is it called Python?

When he began implementing Python, Guido van Rossum was also reading the published scripts from “Monty Python’s Flying Circus”, a BBC comedy series from the 1970s. Van Rossum thought he needed a name that was short, unique, and slightly mysterious, so he decided to call the language Python.