Python quick reference

Variables and Types

object oriented, no need to declare variables or declare their type.

• numbers
• strings
• lists

variable operations

assigenment examples

simple string assignment

 >>> var1 = 'foo'

this automatically creates a variable of type string

>>> type(var1)
<type 'str'>

printing your variable (python 3 requires parenthesis)

 >>> print(var1)
 foo

string assignment like above must be incapsulated by quotes or the right side is interpretted as a variable name.

example:

>>> var1 = foo
Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: name 'foo' is not defined

integer variable

>>> my_int_var1 = 1
>>> print my_int_var1
1

assigning a variable to another variable

>>> var2 = 'bar'
>>> var1 = var2
>>> print var1
bar

muli-variable example:

>>> var1 = 'foo'
>>> var2 = 'bar'
>>> new_var = "%s %s" % (var1,var2)
>>> print new_var
foo bar

print

print sing variable named foo

>>> print(foo)

print variables with text

print 'my variable is %s' % FOO

or reference multiple vars with a tuple

print 'my variable are %s %s' % (FOO, BAR)

print without a newline, use comma at the end of the print statement

for i in range(10):
...     print i,
... else:
...     print
...
0 1 2 3 4 5 6 7 8 9

Common string operations

print nth word of string

 print s.split()[n]

str = "this is string example....wow!!!"
print (str.split( ))
print (str.split('i',1))
print (str.split('w'))

>>> print (str.split( ))[0]
this

string replace

string = "geeks for geeks geeks geeks geeks"

Prints the string by replacing geeks by Geeks

print(string.replace("geeks", "Geeks"))  
 

Prints the string by replacing only 3 occurrence of Geeks

print(string.replace("geeks", "GeeksforGeeks", 3))

grep equivalent

use splitlines and test for string using regex via re.search

import re
txt = "foo\nbar\n foo\n bar"
for line in txt.splitlines():
   if re.search("\sfoo", line):
     print line

Control flow constructs

for condition

for x in mylist:
   print(x)

Basic Operators

Arithmetic Operators

addition, subtraction, multiplication, division: +, -, *, /

integer remainder of the division 11 % 3 = 2

power relationships, ie: 2 to the power or 3 = 2 ** 3

concatenating strings: helloworld = "hello" + " " + "world"

System-specific parameters and functions

Command line arguments - sys.argv

required...
import sys
usage....
sys.argv[0] = name of script
sys.argv[1] = first arg
sys.argv[x] = xth arg, where x=some integer
len(sys.argv) = number of args
str(sys.argv) = argument list

[ https://medium.com/swlh/python-argparse-by-example-a530eb55ced9 Python Argparse by Example - Rupert Thomas]

https://docs.python.org/3/library/argparse.html

accessing input from a pipe

script that takes input from pipe and outputs it again to stdout

import sys
 
for line in iter(sys.stdin.readline, ''):
    sys.stdout.write(line)