Python Modules

import os

print(os.listdir("."))

from os import listdir

print(listdir("."))

from os import listdir, getcwd

print("Current directory:", getcwd())
print(listdir("."))

from os import listdir as showMeTheFiles

print(showMeTheFiles("."))

### Exercises 1. The `re` Module: Regular Expressions ------------------------------------ Regular expressions are patterns that match all or part of a string, they are very useful in any kind of text processing application and every programmer should understand how and when to use them. The Python `re` module provides support for various operations using regular expressions, this section gives some details of the most useful parts of this module. A regular expression is a pattern specified in a special language where certain symbols affect the way that strings are matched. For example, the '.' character will match any character in a string, so the regular expression `'...'` will match any sequence of three characters, whatever they are. The full language is quite complicated but the most useful parts are simple to master. We'll first look at a simple example pattern and show some of the things you can do with it, then describe the most useful parts of the regular expression language. If I want to find all of the dollar amounts in a text I can write a simple regular expression that matches things like \$4,000, \$3, \$200 etc. (for simplicity, I'm ignoring amounts with a decimal point). Basically I'm looking for a dollar sign, followed by one or more digits, possibly with commas. That last sentence gives a model for the regular expression I need to write, I just need to encode that in the regular expression language: - **A dollar sign**: `'\$'` the dollar sign is special in the regular expression language, it means match at the end of a string, so we need to preceed it with a backslash to just match a dollar. - **followed by one or more digits possibly with commas**: `'\$[0-9,]+'` The square bracket notation matches any character from the set in brackets. In this case I include the digits 0-9 (shorthand for including all the digits) and the comma character. The + sign after the brackets means do this one or more times. So the final pattern we have is `'\$[0-9,]+'`, this should match any dollar amount in a text. Let's see how to apply this to search for occurrences of dollar amounts in a text. The `re.search` procedure finds matches to a pattern in a string. The returned value from the `re.search` method is a match object which contains details of the matching text. If there is no match then it returns `None`. So, the first use of this procedure is to simply test whether the pattern is present or not, since None evaluates to False and the match object to True in a test, we can write:

import re

def string_has_money(text):
    """Return True if this text string
    contains a dollar amount, False otherwise."""

    dollar_re = r'\$[0-9,]+'
    if re.search(dollar_re, text):
        return True
    else:
        return False

Next I might want to find what the dollar amounts are in a string. For this I need to interrogate the return value of the `re.search` procedure. As I said, this is an object and we can call the `group()` method to find what was matched:

import re

def get_money_from_string(text):
    """Return the first dollar amount from the
    text string or None if none is found."""

    dollar_re = r'\$[0-9,]+'
    match = re.search(dollar_re, text)
    if match:
       return match.group()
    else:
       return None

I might also want to find all of the matches to this pattern. There are two ways to achieve this. The procedure `re.findall` returns a list of strings that match the pattern and the procedure `re.finditer` allows you to iterate over the matches with a for loop. So, I can find all dollar amounts like this:

import re

def get_all_money_from_string(text):
    """Return a list of the dollar amounts from the
    text string or the empty list if none is found."""

    dollar_re = r'\$[0-9,]+'
    return re.findall(dollar_re, text)

Finally, I might want to censor a text by replacing any dollar amount with \$\$\$\$CAPITALISM\_SUCKS!!!!. I could do this with the `re.sub` procedure:

import re

def censor_text(text):
    """Replaces any dollar amount in the text
    string with a suitable anti-capitalist message.
    Returns the resulting string."""

    dollar_re = r'\$[0-9,]+'
    message = "$$$$CAPITALISM_SUCKS!!!!"
    return re.sub(dollar_re, message, text)

These are just a few of the things you can do with the regular expression library. In particular the match object that is returned by `re.search` has many more capabilities that are useful for more complicated patterns. ### More on Regular Expression Patterns Here are the most useful parts of the regular expression language. A more complete reference can be found [in the Python documentation](http://docs.python.org/library/re.html#regular-expression-syntax). '.' : Matches any character except newline '\[\]' : Used to indicate a set of characters and matches any one of those characters. You can include ranges like \[a-z\], \[0-4\], \[A-F\] as well as explicit sets like \[abc\]. Special characters like . and + lose their meaning inside of square brackets so \[0-9.\] matches either a digit or a period. '\\d' : Matches a decimal digit, equivalent to '\[0-9\]' '\\s' : Matches any whitespace character, space, tab or newline etc. '\\S' : Matches any non-whitespace character, the opposite of \\s. '\\w' : Matches any alphanumeric character or underscore, equivalent to \[A-Za-z0-9\_\]. '\*' : A modifier which means that the previous pattern matches zero or more times. '+' : A modifier which means that the previous pattern matches one or more times. '?' : A modifier which means that the previous pattern is optional (matches zero or one time).