Lesson 7: How to Use Help, List

and Dictionary Methods

Fundamentals of Text Processing for Linguists

Na-Rae Han

Objectives

Learning on your own

dir(), help()

Python IDLE tooltips

Using online references

List methods

Dictionary methods

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Teaching yourself new tricks

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Python built-in helper functions

dir()

help()

Python IDLE tool tips

Online references

Python 2.7 Quick Reference:

http://rgruet.free.fr/PQR27/PQR2.7.html

>>> range(

dir() and help()

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dir(obj) Returns a list of

attributes (__xyz__) and methods that are available

for the given object.

>>> dir(str) ['__add__', '__class__', '__contains__', '__delattr__', ... '__subclasshook__', '_formatter_field_name_split', 'capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

dir() and help()

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>>> dir(str) ['__add__', '__class__', '__contains__', '__delattr__', ... '__subclasshook__', '_formatter_field_name_split', 'capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

>>> help(str.find) Help on method_descriptor: find(...) S.find(sub [,start [,end]]) -> int Return the lowest index in S where substring sub is found, such that sub is contained within S[start:end]. Optional arguments start and end are interpreted as in slice notation. Return -1 on failure.

help(obj.method) prints out information on

the object's method

Self-learn!

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Using the various sources, find out what the following string methods do:

5 minutes

>>> dir(str) ['__add__', '__class__', '__contains__', '__delattr__', ... '__subclasshook__', '_formatter_field_name_split', 'capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

Try help(str.strip)

Additional string operations (1)

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.isalpha() returns True only if all characters

are alphabetic

.isalnum() returns True only if all characters

are a digit or an alphabet

.isdigit() returns True only if all characters

are a digit

.isspace() returns True only if all characters

are a whitespace character

>>> 'co-operate'.isalpha() False >>> 'Exercise2'.isalnum() True >>> '2013'.isdigit() True >>> ' \n\t'.isspace() True

>>> ' green ideas \n'.strip() 'green ideas'

>>> 'green ideas'.find('e') 2 >>> 'green ideas'.find('ea') 8 >>> 'green ideas'.find('t') -1 >>> 'green ideas'.count('e') 3 >>> 'green ideas sleep'.count('ee') 2 >>> 'The thirty-three thieves thought that'.count('th') 5

Additional string operations (2)

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.strip() returns a string stripped of whitespaces on either edge

.find() searches for the given string

within str, and returns the first index where it begins.

Returns -1 if not found.

.count() searches for the given string and

returns the total count

List operations

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List methods

Functions that are defined on the list datatype

Called on a list object, has this syntax:

listobj.method()

Lists are mutable, which means list methods modify the caller object (list) in place.

>>> li = [8, 'abc', 4.5, 11]

>>> li[2]

4.5

>>> li[2] = 1000

>>> li

[8, 'abc', 1000, 11]

Lists are mutable

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We can change individual list elements

These elements are changed in place: the rest of the list is not affected

The list name 'li' still points to the same memory reference when we're done.

Because lists are mutable, they are not as fast as tuples.

Tuples are immutable

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You can't change a tuple.

Instead, what you should do is make a fresh new tuple and reassign the name:

>>> tu = ('Spring', 'Summer', 'Fall', 'Winter') >>> tu[2] 'Fall' >>> tu[2] = 'Autumn' Traceback (most recent call last): File "<pyshell#20>", line 1, in <module> tu[2] = 'Autumn' TypeError: 'tuple' object does not support item

assignment

Adding to a list

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>>> li = [1,2,3]

>>> li

[1, 2, 3]

>>> li.append(4)

>>> li

[1, 2, 3, 4]

>>> li.extend([5,6,7])

>>> li

[1, 2, 3, 4, 5, 6, 7]

>>> li.insert(1, 1.5)

>>> li

[1, 1.5, 2, 3, 4, 5, 6, 7]

Try dir(li) and help()! ??

??

??

>>> li = [3, 9, 'ab', 3.5]

>>> li.append('a')

>>> li

[3, 9, 'ab', 3.5, 'a']

>>> li.extend([9, 11, 'c'])

>>> li

[3, 9, 'ab', 3.5, 'a', 9, 11, 'c']

>>> li.insert(2, 'x')

>>> li

[3, 9, 'x', 'ab', 3.5, 'a', 9, 11, 'c']

List methods

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.append(x) adds a single item

at the end

.extend(list) adds a list of items

at the end

.insert(i,x) inserts an item

at index i

>>> li = [1, 2]

>>> li.append(3)

>>> li

[1, 2, 3]

>>> li.extend([4,5])

>>> li

[1, 2, 3, 4, 5]

>>> li.append([6,7])

>>> li

[1, 2, 3, 4, 5, [6, 7]]

>>> len(li)

6

.append() vs. .extend()

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List inside a list! [6,7] is appended

as a single element. li has length 6

.extend() vs. +

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+ also extends a list, but it creates and returns

a NEW list. li is NOT affected.

>>> li = [1, 2, 3]

>>> li.extend([4, 5, 6])

>>> li

[1, 2, 3, 4, 5, 6]

>>> li + [7, 8, 9]

[1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> li

[1, 2, 3, 4, 5, 6]

.extend() vs. +

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+ also extends a list, but it creates and returns

a NEW list. li is NOT affected.

>>> li = [1, 2, 3]

>>> li.extend([4, 5, 6])

>>> li

[1, 2, 3, 4, 5, 6]

>>> li + [7, 8, 9]

[1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> li

[1, 2, 3, 4, 5, 6]

>>> li = li + [7, 8, 9]

>>> li

[1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> li += [10, 11]

>>> li

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

To extend li itself, reassign it to the

new, returned list.

List methods based on item value

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.index(x) index of first occurrence

.count(x) number of occurrences

.remove(x) remove first occurrence only

>>> li = ['a', 'b', 'c', 'b']

>>> li.index('b')

1

>>> li.count('b')

2

>>> li.remove('b')

>>> li

['a', 'c', 'b']

>>> li = ['a', 'b', 'c', 'b']

>>> li.index('b')

1

>>> li.count('b')

2

>>> li.remove('b')

>>> li

['a', 'c', 'b']

List methods based on item value

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Careful – These throw an error

if 'b' is not found in the list

Use in conjunction with the in operator:

if 'b' in li : li.remove('b')

.pop()

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>>> li = ['a', 'b', 'c', 'd', 'e']

>>> li.pop()

'e'

>>> li

['a', 'b', 'c', 'd']

>>> li.pop(2)

'c'

>>> li

['a', 'b', 'd']

removes the last item from the list

and returns it

removes the item at index

and returns it

.pop()

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>>> li = ['a', 'b', 'c', 'd', 'e']

>>> li.pop()

'e'

>>> li

['a', 'b', 'c', 'd']

>>> li.pop(2)

'c'

>>> li

['a', 'b', 'd']

.pop() removes an item

from the list; list no longer

contains the item

.pop()

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>>> li = ['a', 'b', 'c', 'd', 'e']

>>> li.pop()

'e'

>>> li

['a', 'b', 'c', 'd']

>>> li.pop(2)

'c'

>>> li

['a', 'b', 'd']

Because the popped item 'c' is returned, you can assign a name to it, e.g.,

x = li.pop(2) x's value is 'c'

.pop() vs. .append()

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>>> li = ['a', 'b', 'c']

>>> li.append('x')

>>> li

['a', 'b', 'c', 'x']

>>> li.pop()

'x'

>>> li

['a', 'b', 'c']

.append('x')

& .pop()

undo each other

.pop() vs. .insert()

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>>> li = ['a', 'b', 'c']

>>> li.insert(2, 'x')

>>> li

['a', 'b', 'x', 'c']

>>> li.pop(2)

'x'

>>> li

['a', 'b', 'c']

.insert(i,'x') &

.pop(i) undo each other

Practice

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add 'thou' to the list

change 'i' to "I'

add 'we' and 'they'

remove 'thou'

add pron2 to pron

add 'yinz' between 'we' and 'they'

Try it yourself

2/19/2014

2 minutes

dict: a dictionary data type

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A dictionary of the Simpson family members' age

A dictionary of verb past tense

Dictionaries store a mapping between a set of keys and a set of values.

Keys can be any immutable type: string, integer, tuple

Values can be any type (can also be mixed types)

There is no inherent order (unlike lists and tuples)

You can define, modify, view, lookup, and delete the key-value pairs in the dictionary.

{'Homer':36, 'Marge':36, 'Bart':10, 'Lisa':8, 'Maggie':1}

{'go':'went', 'eat':'ate', 'see':'saw', 'say':'said'}

Looking up a dictionary

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>>> en2es = {'cat':'gato', 'dog':'perro', 'tiger':'tigre'}

>>> en2es['cat']

'gato'

>>> en2es['dog']

'perro'

>>> en2es['gato']

Traceback (most recent call last):

File "<pyshell#2>", line 1, in <module>

en2es['gato']

KeyError: 'gato'

Dictionary is one way. Cannot look up based on

the value. Mapping can be many-to-

one.

Adding and deleting an entry

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>>> en2es = {'cat':'gato', 'dog':'perro'}

>>> en2es['cat']

'gato'

>>> en2es['tiger'] = 'tigre'

>>> en2es

{'tiger': 'tigre', 'dog': 'perro', 'cat': 'gato'}

>>> del en2es['dog']

>>> en2es

{'tiger': 'tigre', 'cat': 'gato'}

There is no order in dictionary!

del deletes a key and its value from a

dictionary

creates a new key & maps value

Checking if something's in a dictionary

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>>> en2es

{'tiger': 'tigre', 'dog': 'perro', 'cat': 'gato'}

>>> 'fox' in en2es

False

>>> 'cat' in en2es

True

>>> 'gato' in en2es

False

"in" does not work with value

"in" tests if a key is in a dictionary

Finding out what's in

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>>> en2es

{'tiger': 'tigre', 'wolf': 'lobo', 'cat': 'gato'}

>>> en2es.keys()

['tiger', 'wolf', 'cat']

>>> en2es.values()

['tigre', 'lobo', 'gato']

>>> en2es.items()

[('tiger', 'tigre'), ('wolf', 'lobo'), ('cat', 'gato')]

.keys() returns a list of keys, .values() returns a list of values.

The orders match!

A list of key, value TUPLES ('pairs')

Iterating through dictionary

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>>> en2es

{'tiger': 'tigre', 'wolf': 'lobo', 'cat': 'gato'}

>>> 'tiger' in en2es

True

>>> for k in en2es :

print k, 'is', en2es[k]

tiger is tigre

wolf is lobo

cat is gato

en2es.keys() also works

"in" tests if a key is in a dictionary

Iterating through key:value tuples

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>>> en2es

{'tiger': 'tigre', 'wolf': 'lobo', 'cat': 'gato'}

>>> en2es.items()

[('tiger', 'tigre'), ('wolf', 'lobo'), ('cat', 'gato')]

>>> for (k,v) in en2es.items() :

print k, 'is', v

tiger is tigre

wolf is lobo

cat is gato

Common dict application: counting

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>>> tally = {'gold':1, 'bronze':3}

>>> tally['bronze']

3

>>> medals = ['bronze', 'silver', 'gold', 'gold', 'silver']

>>> for m in medals:

tally[m] += 1

Traceback (most recent call last):

File "<pyshell#80>", line 2, in <module>

tally[m] += 1

KeyError: 'silver'

Error: 'silver' is not yet in the

dictionary as a key

Common dict application: counting

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>>> tally = {'gold':1, 'bronze':3}

>>> tally['bronze']

3

>>> medals = ['bronze', 'silver', 'gold', 'gold', 'silver']

>>> for m in medals:

if m not in tally :

tally[m] = 1

else :

tally[m] += 1

>>> tally

{'bronze': 4, 'gold': 3, 'silver': 2}

Make sure to account for the

initial key creation & assignment

Word count

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sent = 'Rose is a rose is a rose is a rose.' words = sent.split() counts = {} for w in words : if w in counts : counts[w] += 1 else : counts[w] = 1 print counts

>>> {'a': 3, 'Rose': 1, 'is': 3, 'rose.': 1, 'rose': 2} >>>

Fold case We really must start tokenizing

punctuation.

Word count and tokenization

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sent = 'Rose is a rose is a rose is a rose.' words = sent.lower().replace('.',' .').split() counts = {} for w in words : if w in counts : counts[w] += 1 else : counts[w] = 1 print counts

>>> {'a': 3, 'rose': 4, 'is': 3, '.': 1} >>>

'.' is its own word. 4 tokens of 'rose'

Word count and tokenization

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sent = 'Rose is a rose is a rose is a rose.' words = sent.lower().replace('.',' .').split() counts = {} for w in words : if w in counts : counts[w] += 1 else : counts[w] = 1 print counts

>>> {'a': 3, 'rose': 4, 'is': 3, '.': 1} >>>

'.' is its own word. 4 tokens of 'rose'

2 minutes

Wrap-up

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Next class

Sorting

File IO: reading from and writing to a file

Exercise5

Due Tuesday midnight

Not yet online – will be up tonight