Python for System Administrators

# before starting this lesson
#  import the python3 print capabilities
#  using the following statement
#  NB since now you *must* always use
#     parenthesis with print!
from __future__ import print_function, unicode_literals, division
import sys

# Import further python3 features.
if sys.version_info.major < 3:
  from future_builtins import *
  import __builtin__ as builtins

Introducing Lists

# it's easy to create a list
list_a = ['this', 'is', 'a', 'list']

# you can append items to a list
# with the append method
list_a.append("mutable")

# Print its content
print(list_a)

# See its length
len(list_a)

Lists and Generators

a = 11
#range in python 2 returns a list
# of consecutive ints
from_0_to_10 = range(a)
l = len(from_0_to_10)
l == a
print(l)

# in python 3 things are slightly different
# so the above code won't work.
import sys
if sys.version_info.major < 3:
    range = xrange  # Override the range function with the xrange function.
from_0_to_10 = range(a)

# Exercise: can you print the content of from_0_to_10
print(from_0_to_10)

# In python 3, you need to create a list from the range object.
# The above code
# should be replaced with the following
from_0_to_10 = list(range(a))
print(from_0_to_10)

Getting list items

# you can get list items by index
from_0_to_10[0]

# Exercise: what happens if I get the 11th element?
from_0_to_10[11]

# python is calling under the hood
from_0_to_10.__getitem__(0)

# python lists are doubly linked ;)
from_0_to_10[-1]

#  Exercise: please check the manual!
help(list)

Slicing a list

straight = [1, 2, 3, 'star']
print(straight)

# I can take the middle of the list...
straight[1:3]

k = 2  # ... or using a separator
straight[0:k], straight[k:4]

straight[:k]         # I can omit the first...

straight[k:]       # ...and last index

Strings and Lists

# Strings behaves like lists
s_a = "Counting: 123"

# Have length..
l_a = len(s_a)
print(l_a)

# ..indexes
print(s_a[0], " ", s_a[-1])

# and a last element
s_a[l_a] # ...what's happening there?

# ...we can even slice them
f = "prova.txt"
f[:-4], f[-4], f[-3:]

# Note: use the os.path module for path manipulation.
# The above code will break with 4-letter extensions!
from os.path import splitext
splitext("prova.foo.yaml")

Iterations: for

a_list = ['is', 'iterable', 'with']
for x in a_list:
    print(x)

for x in a_list:
    # You need `from __future__ import print_function`
    # for python2 to support the `end` argument
    print((x), end=' ')
    y = x + str(2)
    break       # stop now

# what's the expected output of the
# following instruction?
print(("x,y: ", (x, y)))
# Differently from C, `for` does not create
#  a scope

Iterations: while

a_list = ['is', 'iterable', 'with']
while a_list:
    # pop() modifies a list removing
    #  and returning its last element
    x = a_list.pop()
    print(("pop out %s" % x))
    break  # what happens if I remove this break?

print(a_list)

# What's the expected behavior of the
#  following instructions?
for x in a_list:
    print((x + a_list.pop()))
# for + pop() is not always a good idea ;)