Python Programming for Biology: Bioinformatics and Beyond

Note: above lines are for

Python 2.

print(‘Hello’)

Note: above line is for Python

2 or 3.

print(‘Data line’, file=fileObj)

Note: above line is for Python

3.

raise

Manually triggers an error

exception. May be used on its own

for custom exceptions or to re-

throw error objects caught with try:

and except:.

if x == 0.0:

raise

Exception("Detected zero")

---

return

Exits from a named function and

optionally passes back one or more

items to the point where the

function was called.

def countWords(text):

n = len(text.split())

return n

print(countWords('Hello

world'))

try

Encapsulates a block of code so that

if an illegal state triggers an

exception the error can be caught

and dealt with in a special way.

Used in conjunction with except

and or finally keywords.

try:

x += a / b

except ZeroDivisionError:

print("Ignored zero

division")

while

Continues the repeated, iterative

execution of a block of code while

a certain condition holds.

x = 2

while x < 1000:

print(x)

x *= 2

with

Encapsulates a block of code using

a context manager object, which

has dedicated methods to deal with

starting and cleanly ending the

context. Generally used so that the

context can have clean-up before it

exits or an exception occurs.

Optionally uses the as keyword.

with open(fileName) as

fileObj:

for line in fileObj:

print(line)

The with statement

encapsulates the file reading

block so the file is closed at

the end or if an error occurs.

yield

Used inside a function to pass back

a value like return, but allows the

function to be re-entered, keeping

the previous state. Used to make

generator functions.

def generateSquare():

for x in range(1,10):

yield x*x

print(generateSquare())

# 1

print(generateSquare())

# 4