Python tips

List of python tips

1. Reverse a string or list
2. Reverse by custom step
3. List slice assignment
4. Copy a list
5. Create a list out of string
6. Check file or directory exists
7. Call an external command
8. Ternary conditional operator
9. else in for loop
10. Print to file

Reverse a string or list

my_list = ['a','b','c','d','e']
reverse_list = my_list[::-1]

my_string = "python"
print my_string[::-1]

Reverse by custom step

my_list = [1,2,3,4,5,6]
reverse_list = my_list[::-2]
# output : [6,4,2]

List slice assignment

  my_list = ['a','b','c','d','e']
  my_list[3:] = ['x','y','z']
  print my_list
  #output : ['a', 'b', 'c', 'x', 'y', 'z']

Copy a List

a = [1, 2, 3, 4]
''' Considered as one of the weirdest syntax to copy elements.'''
a_copy = a[:]

''' Another way of copying a list.'''
a_copy2 = list()
a_copy2.extend(a) # output a_copy2 = [1, 2, 3, 4]

Create a list out of string

data = "abcd"
data_list2 = list(data)  # OutPut: data_list = ['a', 'b', 'c', 'd'] 

Check file or directory exists

os.path.isfile used only for files

import os.path
os.path.isfile(filename) # True if file exists
os.path.isfile(dirname) # False if directory exists

os.path.exists used for both files and directories

import os.path
os.path.exists(filename) # True if file exists
os.path.exists(dirname) #True if directory exists

Call an external command

from subprocess import call
call(['ls,'-l'])

Ternary conditional operator

print 'True' if True else 'False'

else in for loop

An else block is executed if the loop body is not terminated by a break statement :

for i in range(5):
    print('Here we go!')
    if i == 2:
        break
else:
    print('There we went.')
else :
    print 'String is empty'
# output : Here we go!
#          Here we go!
#          Here we go!

This for loop will get all the way to else:

for i in range(5):
    print('Here we go!')
    if i == 10:
        break
else:
    print('There we went.')
# output : Here we go!
#          Here we go!
#          Here we go!
#          Here we go!
#          Here we go!
#          There we went.

Print to file

output = open('file.txt','w')
print >>output, "Python!!"