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Python programs

How can I run a program? Input and output

In this module you can learn

  • How to read, process, and output text
  • How to read from the keyboard
  • How to write to the screen
  • How to repeat things
  • How to create your own modules

Challenge

Game of instruction

You will be given the drawing of a shape and in group you should write instructions to draw the same shape from scratch The instructions will be given to another group that haven't seen the shape and should draw it following the instructions

Challenge

Questions for getting started (given to small groups on cards)

  • Write examples of 4 programs you know
  • What do programs do?
  • Write a definition of program.

slot

What is a program?

It is a text file that contains Python commands or, in other words, lines of code

Challenge #1

  • Open a text file, write:
  • print "This is the output of my first program"
  • save the file with the name my_print.py and exit
  • Open a terminal, go to the directory where you saved my_print.py and type at the cursor: python my_print.py


Challenge

Your first Python commands: create pairs

slot

> ---

How programs work

slot

Input

slot

Input from the program itself

a = 3
print a

Input from the keyboard

>>> a = raw_input("Type a number: ")
Type a number: 3
>>> print a
3

Challenge #2

Write a program that reads something from the keyboard and print it to the screen.

See the solution to challenge #2

Input from a text file

  • We need to “access” an existing input file
  • And read its content
Infile = open("insulin.txt")

content = Infile.read()

print content

From a Python module

  • A Python module is a text file (with the .py extension) that contains (Python) definitions/assignments
  • Python modules can be accessed from programs using the import statement

Python module insulin.py

insulin = "GIVEQCCTSICSLYQLENYCNFVNQHLCGSHL\
VEALYLVCGERGFFYTPKT"

Python program my_first_import.py

from insulin import insulin
print insulin

Challenge #3

Write a program that reads a sequence from a file and print it to the screen. Run it.


Output

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To the computer screen

Which command we use?

To a text file

  • We need to “open” a text file in the “writing” mode
  • We have to write to it.
from insulin import insulin

outfile = open("my_output.txt", "w")
outfile.write(insulin)
outfile.close()


>Challenge --- >Your first Python commands: create pairs

slot

> ----

Challenge #4

Calculate DNA base occurrences

Write a program that counts how many times the four bases occur in a DNA sequence. The program should:

  • Store the DNA sequence in a variable.
  • Count how often each base occurs.
  • Write all four numbers to the screen. Test it with a DNA sequence for which you know the result, for instance “AAAACCCGGT”. This approach makes it much easier to discover small program errors.

See the solution to challenge #4

Recap

  • string variables contain text
  • print writes to the screen
  • you can use functions to do things
  • you can enter text with raw_input()
  • write() writes to an open file
  • for loops repeat commands
  • comments starts with # or '''