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Built-In Functions

We have already met some of the built-in functions

Let's discuss a few more

File I/O

First we look at some built-in functions for file input/output

All these functions are for operating on text files

Opening and Reading Files

Suppose I have a file test.txt in the current working directory

Contents of the file is 

Foo foo
Bar bar

I can read this in using the built-in function open(): 

>>> f = open('test.txt', 'r')
>>> text = f.read()
>>> text
'Foo foo\nBar bar\n'
>>> print text
Foo foo
Bar bar
>>> f.close()

The arguments of open() are the filename and I/O type (here it's r for read)

A call to open() returns a file object 

>>> f = open('test.txt', 'r')  # Bind f to a new file object
>>> type(f)
<type 'file'>

Just as lists, strings, etc. have methods, so do file objects

Paths

Note that if test.txt is not in current directory then open() fails

In the next example, test.txt is in /tmp 

>>> import os
>>> os.getcwd()   # Current directory is my home directory
'/home/john'
>>> f = open('test.txt', 'r')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: [Errno 2] No such file or directory: 'test.txt'

In this case I have two choices 

>>> f = open('/tmp/test.txt', 'r')  # This call succeeds

>>> os.chdir('/tmp')
>>> f = open('test.txt', 'r')

It will be different on Windows/Mac, but I'm sure you can figure it out.

The readline() Method

To read in individual lines use readline() 

>>> f = open('test.txt', 'r')
>>> f.readline()
'Foo foo\n'
>>> f.readline()
'Bar bar\n'
>>> f.readline()     # Continues to return the empty string
''

To read all lines into a list of strings use readlines() 

>>> f = open('test.txt', 'r')
>>> f.readlines()
['Foo foo\n', 'Bar bar\n']

Note the difference between read() and readlines()

Finally, we can step through the lines in a for loop as follows 

>>> f = open('test.txt', 'r')
>>> for line in f:
...  print line

This looks a bit magical, but we'll learn how it works in the lecture on iterators

Writing to Files

In order to write to files, we use the write() method 

>>> f = open('newfile.txt', 'w')   # Open for *writing* using 'w'
>>> f.write('jailbreak\n')
>>> f.write('by AC/DC\n')
>>> f.close()
>>> f = open('newfile.txt', 'r')   # Next I open for reading using 'r'
>>> print f.read()
jailbreak
by AC/DC

Be careful: if file.txt exists, then the call open('file.txt', 'w') will overwrite it

Other Built-Ins

Let's quickly mention some other useful built-in functions

all() and any()

Two useful built-in functions are any() and all()

Given a list or tuple X, 

>>> X = [False, True, True]
>>> all(X)
False
>>> any(X)
True

Elements don't need to be bools 

>>> X = ['a', 'b', '']
>>> all(X)
False

zip() and enumerate()

These are built-in functions, mainly used for looping 

countries = ['Japan', 'Korea', 'China']
cities = ['Tokyo', 'Seoul', 'Beijing']

for country, city in zip(countries, cities):
    print 'The capital of %s is %s' % (country, city)

X = ['a', 'b', 'c']

for index, value in enumerate(X):
    print 'X[%i] = %s' % (index, value)

These two built-ins are used a lot, so please remember them

Exercises

Exercise 1

Given two numeric lists/tuples X and Y of equal length, compute their inner product. Use zip()

Exercise 2

Consider the polynomial



Let the list of coefficients be given by coeff = [1, 3, -2, 9, 0, 1]

Using enumerate(), evaluate the polynomial at x = 7

Solutions

Solution to Exercise 1 

print sum([x * y for x, y in zip(X,Y)])

Incidentally, this also works 

print sum(x * y for x, y in zip(X,Y))

Solution to Exercise 2 

coeff = [1, 3, -2, 9, 0, 1]
x = 7 

print sum(a * x**i for i, a in enumerate(coeff))