Python 2.7 syntax examples:
Code samples at : https://github.com/rwatsh/python
1. Math:
2**3 = 8 , where, ** is exponent operator
+, -, *, / = math operators
% = modulo operator
2. Comments:
# - single line
comment
“”” my multi line
comment “””
3. Print:
print(“%.2f”, total)
-> will print total rounded to 2 decimal places.
4. Escape char:
'There\'s a snake in
my boot!'
Not required when using double quotes for string.
5. String:
fifth_letter =
"MONTY"[4]
print fifth_letter => prints
Y
parrot = "Norwegian Blue"
print len(parrot)
print parrot.lower()
print parrot.upper()
pi = 3.14
print str(pi) => str() converts
non-strings to string
Methods that use dot notation only work with
strings.
On the other hand,
len() andstr() can work on other data
types.
print "Spam " +
"and " + "eggs"
print "The value of pi is
around " + str(3.14)
string_1 = "Camelot"
string_2 = "place"
print "Let's not go to %s.
'Tis a silly %s." % (string_1, string_2)
ð Let's not go to Camelot. 'Tis a
silly place.
String slicing:
s = "Charlie"
print s[0]
# will print "C"
print s[1:4]
# will print "har"
Getting i/p and printing o/p:
name = raw_input("What is your name?")
quest = raw_input("What is your quest?")
color = raw_input("What is your favorite
color?")
print "Ah, so your name is %s, your quest is %s,
" \
"and your favorite color is %s." % (name,
quest, color)
6. Datetime library:
from datetime import datetime
print datetime.now() => 2014-07-25
17:42:13.304805
print now.month => 7
print now.day => 25
print now.year => 2014
print '%s/%s/%s' % (now.month,
now.day, now.year) => 7/25/2014
print '%s:%s:%s' % (now.hour,
now.minute, now.second) =>17:48:28
7. Comparison:
==, !=, <, >, <=, >=
8. Logical: and, or, not
9. Conditional: if, elif, else
def clinic():
print ("You\'ve just entered the
clinic!")
print ("Do you take the door on the
left or the right?")
answer = input("Type left or right and
hit 'Enter'.").lower()
if answer == "left" or answer ==
"l":
print ("This is the Verbal Abuse
Room, you heap of parrot droppings!")
elif answer == "right" or answer
== "r":
print ("Of course this is the
Argument Room, I've told you that already!")
else:
print ("You didn't pick left or
right! Try again.")
clinic()
clinic()
10. Functions:
See above example –
def [function_name(param1,
param2)]:
x = 2
return x
11.
Import:
a. Generic import: Import a module by name.
Ex:
import math
print math.sqrt(25)
b. Function import:
from [module] import
[function]
from math import
sqrt
print sqrt(25)
c. Universal import:
from
[module] import *
from math import *
print sqrt(25)
d. Check everything a module has:
Import math
Print dir(math)
12. Built-in functions: max, min, abs, type
Max(1,2,3) => 3
Min(1,2,3) => 1
Abs(-10) => 10
Print type(1) => or type(num) == int
13. List:
zoo_animals =
["pangolin", "cassowary", "cat", "dog"]
print "The
first animal at the zoo is the " + zoo_animals[0]
print len(zooanimals)
print
zooanimals[0:2] => 1st and 2nd elements in the list
pets = zooanimals[2:4]
or zooanimals[2:] => cat and dog => slicing the list.
Cat_index = zooanimals.index(“cat”)
Zoonimals.insert(cat_index,
“cobra”) => ["pangolin", "cassowary", “cobra”, "cat",
"dog"]
Zooanimals.sort()
=> sort the list.
zooanimals.remove(“cat”)
=> remove element from list
pets[] => empty
list
print “----“.join(zooanimals)
=> pangolin ---- cassowary ---- cobra ---- cat ----- dog
List Comprehension syntax:
my_list = range(51)
evens_to_50
= [i for i in range(51) if i % 2 == 0]
List Slicing:
List slicing allows us to access elements of a list in a concise manner.
The syntax looks like this:
[start:end:stride]
my_list = range(1, 11) # List of
numbers 1 - 10
# print all odds
from start to finish so no need to specify the start and end only need stride.
print my_list[::2]
# reverse a list
backwards =
my_list[::-1]
to_21 = range(1,22)
odds = to_21[::2]
middle_third =
to_21[7:14:1]
14. For loop:
my_list = [1,9,3,8,5,7]
for number in my_list:
print 2*number
else:
print “Well done”
The else block only executes if
the for loop executes normally (ie there is no break causing the loop to
terminate mid-way).
for loop with range():
n = [3, 5, 7]
def print_list(x):
for i in
range(0, len(x)):
print x[i]
print_list(n)
for I in range(20) => 0 to 20
for I in range(1, 10) => 1 to 9
Need index with for each loop – use enumerate() built-in function:
choices = ['pizza', 'pasta', 'salad', 'nachos']
print 'Your choices are:'
for index, item in enumerate(choices):
print index+1,
item
Iterate over 2 or more lists at once:
Zip() will create pairs of elements when passed two
lists, and will stop at the end of the shorter list.
list_a
= [3, 9, 17, 15, 19]
list_b
= [2, 4, 8, 10, 30, 40, 50, 60, 70, 80, 90]
for a,
b in zip(list_a, list_b):
# Add your code here!
print max(a,b)
Populate a list:
evens_to_50 = [i for i in range(51) if i % 2 == 0]
print evens_to_50
15. Dictionary:
residents =
{'Puffin' : 104, 'Sloth' : 105, 'Burmese Python' : 106}
menu = {}
menu[‘Samosa’] =
1.60
del residents[‘Puffin’]
=> remove item from dictionary
menu[‘Samosa’] =
1.50
for key in residents
print residents[key]
print residents.items()
=> returns dictionary as list of key/value pairs.
Print residents.keys()
=> list of all keys
Print residents.values()
=> list of all values
16. While loop:
while count < 10:
count += 1
else:
print “Game over!”
17. Print:
The
, character after our printstatement
means that our next printstatement
keeps printing on the same line.
d =
{'a': 'apple', 'b': 'berry', 'c': 'cherry'}
for key
in d:
# Your code here!
print key, " ", d[key]
18. Lambda: Anonymous function:
Python’s support for functional programming
=> meaning you are allowed to pass functions around just as if they were
variables or values.
lambda x: x % 3 == 0
is same as:
def by_three(x):
return x % 3 == 0
Usage: filter uses lamba expression below to print only those elements of
the list that satisfy that return true for the lambda expression (or satisfy
the condition divisible by 3).
my_list = range(16)
print filter(lambda
x: x % 3 == 0, my_list) => [0, 3, 6, 9, 12, 15]
19. Bitwise:
print 5 >> 4 # Right Shift
=> 0
print 5 << 1 # Left Shift
=> 10
print 8 & 5 # Bitwise AND
=> 0
print 9 | 4 # Bitwise OR =>
13
print 12 ^ 42 # Bitwise XOR => 38
print ~88 # Bitwise NOT
=> -89 (equivalent to adding 1 to the number and putting a – sign).
XOR => in-equality is true (0b1100 ^ 0b101010
=> 0b100110)
In
Python, you can write numbers in binary format by starting the number with
0b.
print 0b1 + 0b11 => 4
bin() – to binary (as string)
hex() – to hex
oct() – to octal
print bin(5) => 0b101
print hex(5) => 0x5
print oct(5) => 05
int(str, radix) - any string (includes, binary/hex/octal with
appropriate radix (2,16,8)) to int
int("110", 2) => 6
print int("0b100",2) => 4
print int(bin(5),2) => 5
20. Classes:
An empty class:
class Animal(object):
pass
pass – special python keyword for
placeholder – in areas of code where python expects an expression.
class Triangle(object):
number_of_sides = 3
def __init__(self, angle1,
angle2, angle3):
self.angle1 = angle1
self.angle2 = angle2
self.angle3 = angle3
def check_angles(self):
if self.angle1 +
self.angle2 + self.angle3 == 180:
return True
else:
return False
my_triangle = Triangle(90, 30, 60)
print my_triangle.number_of_sides
print my_triangle.check_angles()
class Equilateral(Triangle):
angle = 60
def __init__(self):
self.angle1 = self.angle
self.angle2 = self.angle
self.angle3 = self.angle
class MyTriangle(Equilateral):
def __init__(self, beauty):
self.beauty = beauty
self.angle1 = 70
my_triangle = MyTriangle(True)
print my_triangle.angle1 # angle1/2/3 not inherited from base class
Triangle
print my_triangle.number_of_sides # member inherited from Triangle
print my_triangle.angle # member inherited from Equilateral
1.
object is base class from which all classes
inherit (same as Java).
2.
Constructors are - __init__(self…)
3.
Self needs to be the first param for all member
methods and constructors.
4.
Derived class constructor needs to initialize
all base members that don’t have a default value.
5.
Derived class inherits all base class members
that have default values (defined outside of base class’s constructor).
21. File I/O:
my_list = [i**2 for i in range(1,11)]
# Generates a list of squares of the numbers 1 - 10
f = open("output.txt", "w")
for item in my_list:
f.write(str(item) +
"\n")
f.close()
or
with open("text.txt", "w") as my_file:
my_file.write("Success!")
with … as syntax is like try-with-resources
in Java and it auto closes the file.
No comments:
Post a Comment