Python Notes

This post conatins Python programming notes, from the Python tutorial that is offered by Google

Basics

• sys.argv[0] ---- program name itself
• arguments and variables similar no types

def Hello(name):
    a = 'hello'

• print seperate by , then space is includes automatically
• print 'Hello',name,a ===> Hello rohan!!! 10
• print'Hello'+name+str(a) ⇒ Hellorohan!!!10
• string not empty, num is not 0 -- > true

How python works

• does everything at last possible second
• in the moment it checks at every line
• strings are immutable (returns new string)
• a.find() - returns first occurance
• a.lower()
• place holder %
• 'Hi %s I have %d donuts' % (‘Alice’,29)
• These are not python strings and not unicode strings these are just series of bytes.

Strings slice

>>> a = 'Hello'
>>> a
'Hello'
>>> a[1:3]
'el'
>>> a[1:2]
'e'
>>> a[1:5]
'ello'
>>> a[1:]
'ello'
>>> a[:5]
'Hello'
>>> a[:]
'Hello'

Negatives

>>> a = 'Hello'
>>> a[-1]
'o'
>>> a[-3:]
'llo'
>>> a[-3:-1]
'll'

Lists

• Same syntax across all
• a = [1,2,3,’a’]
• len(a) to find length
• b = a (does not create a copy both point to same)
• lists are 'mutable'
• python has garbage collector {need to make copies is not much}
• a ==b does the same returns true
• slices works fine with lists as well
• There is built in for each for var in list: print var
  
• Check if value in a list {works for lot of DS - in hash table does }
• value in list
• Built in for lists
• append - does not return new list returns None
• a = [1,2,3]
• a.append(4)
• a = a.append(4) # error NO NO
• a.pop(0) pops of element and returns it
• delete a from local scope
• del a {complete list}
• del a[1] {delete a element}
• The range(n) function yields the numbers 0, 1, ...
• for i in range(100) print i

Sorting

• sorting of lists - don't use .sort()
• sorted - makes a new list and sorts.
• comparison depends on the type of elements compared.{strings,ints,char}

Custom sorting

• old way 2 argument comparator method. python has new way
• sort by length

>>> sorted(a,key=len)
['d', 'a', 'dc', 'aa', 'ccc']
>>> def last(s): return s[-1]

>>> sorted(a,key=last)
['a', 'aa', 'ccc', 'dc', 'd']
split, join
>>> ':'.join(a)
'z:d:dc:a:aa'
>>> '\n'.join(a)
'z\nd\ndc\na\naa'
>>> ':'.join(a)
'z:d:dc:a:aa'
>>> b = ':'.join(a)
>>> b.split(':')
['z', 'd', 'dc', 'a', 'aa']

• Don’t modify list while iterating a list (even java has same constraint)

Tuple

• Fixed size
• immutable
• tup = (1,2,3)
• (x,y,z) coordinates; (url,score) {taking fixed number of strings and pass together)

>>> a = (1,3,3)
>>> a
(1, 3, 3)
>>> len(a)
3
>>> a[0]
1
>>> a[0]=2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
Sort by one thing and then sort by other 
Write key function that returns a tuple
>>> a = [(1,"b"),(2,"a"),(1,"a")]
>>> a
[(1, 'b'), (2, 'a'), (1, 'a')]
>>> sorted(a)
[(1, 'a'), (1, 'b'), (2, 'a')]
Parallel assignment 
>>> (x,y) = (1,2)

HashTable , /Dictionary

• {}
• key,value bindings
• constant time retrieval

>>> d ={}
>>> d['a'] = 'alpha'
>>> d['a']
'alpha'
>>> d['B']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'B'

get(),keys(),items()

d.get()
>>> d.get('a')
'alpha'
>>> d.get('f')
how to find if something is in dict
‘a’ in d
True
.keys()

>>> d.keys()
['a']
>>> d['b'] = 'beta'
>>> d.keys()
['a', 'b']
values()
>>> d.values()
['alpha', 'beta']

looping

>>> for k in sorted(d.keys()): print 'key:',k,'->',d[k]
... 
key: a -> alpha
key: b -> beta
d.items()
    >>> d.items()
[('a', 'alpha'), ('b', 'beta')]
>>> for tuple in d.items(): print tuple
... 
('a', 'alpha')
('b', 'beta')

Files

• creating cat python
• f = open(filename,'r')
• diff modes - 'r','w''rU' text = f.read() lines = f.readlines() f.open(fname,”w”) f.write(text) {be careful we can zero out file}

Regular Expression

• import re
• returns a match object
• match = re.search(pattern,text)
• match = re.search(‘iig’,’its a piiiiiiiig’)
• if match:
• . dot any character except new line --\w word character --\d digit --\s space
• left to right once it finds solution it is done
• if looking for period char >>> Find('a\.','its a. piiiiiiiig') a.
• raw string
  • do not do any special processing with backslashes
• uses for writing regex

\S non whitespace
\s whitespace
\d digit 0-9
\w word character a-z A-Z 0-9

Email example

>>> m = re.search(r'([\w.]+)@([\w.]+)','egrreg gjkenrkj@snfjkv.com erngjk @ ')
>>> m
<_sre.SRE_Match object at 0xb7467800>
>>> m.group()
'gjkenrkj@snfjkv.com'
>>> m.group(1)
'gjkenrkj'
>>> m.group(2)
'snfjkv.com'
>>> m = re.search(r'([\w.]+)@([\w.]+)','egrreg gjken.rkj@snfjkv.com erngjk @ ')
>>> m
<_sre.SRE_Match object at 0xb7467140>
>>> m.group(1)
'gjken.rkj'
>>> m.group(2)
'snfjkv.com'
findall()
    >>re.findall(r'[\w.]+@[\w.]+','egrreg@fmail.com gjken.rkj@snfjkv.com erngjk @ ')
['egrreg@fmail.com', 'gjken.rkj@snfjkv.com']

 re.findall(r'([\w.]+)@([\w.]+)','egrreg@fmail.com gjken.rkj@snfjkv.com erngjk @ ')
[('egrreg', 'fmail.com'), ('gjken.rkj', 'snfjkv.com')]

Flags

• DOTALL also matches newline aswell
• how to use constants >>> m = re.search(r'rohan','ROHAN',re.IGNORECASE) >>> m <_sre.SRE_Match object at 0xb726b3d8> >>> m.group() 'ROHAN'

Modules

• os module
• platform independent systime
• listdir(path) ----> list of strings
  1. import os
os.path.exists(‘/tmp/foo’)
  2. import shutil
shutil.copy(source, dest) ---- >file copying
  3. import commands
     launch a external process and wait for it to finish
     commands.getstatusoutput(cmd)
     returns a tuple of length 2 (status,output) 1st int exit code 2nd is all output std output and std error
• zip -j name all absolute paths

Exceptions

def cat(filename):
  try:
    f = open(filename, 'rU') #U ignare dos line ending unix
    print '-----',filename
    text = f.read() #having just 1 string
    print text
    f.close()
  except IOError:
    print 'No File',filename 
def main():
  args = sys.argv[1:]
  for arg in args:
    cat(arg)

Modularity

• just .py file namespace
• when we load a python file it runs (do not wrie print outside)
  >>> import babynames Hi There !!! >>> dir(babynames) ['__builtins__', '__doc__', '__file__', '__name__', '__package__', 'extract_names', 'main', 're', 'sys'] >>help(babynames.extract_names)

urllib

• Takes url and makes it to look like a file
• import urllib
• Takes a url and tries to look like a file
• uf = urllib.urlopen(‘http://google.com’)
• uf.read()
• has lots of features , set cookies >>> urllib.urlretrieve('http://google.com/intl/en_ALL/images/logo.gif','blah.gif') ('blah.gif', <httplib.HTTPMessage instance at 0xb6fa870c>)