Python: Basic Syntax

16 minute read

Top

Version

I usually assume v3.5 so I will put this after import statements:



1
2
3



   import sys
   # require Python interpreter > v.3.5
   assert sys.version_info >= (3, 5)

Naming

If you get the error module $module has no $method attribute, for example: module csv has no reader attribute when it clearly does then this is because I named my file csv.py. Once renamed to monitorcsv.py, everything worked. Had the same thing happen with email.py.

Modules



1
2
3



   # Placed at top of file under the shebang
   import os
   from random import shuffle # from module import submodule

Variables and Comments and Inspections



1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102



   ################
   # Variables
   # Declare a variable and initialize it, data types are not strong
   >>> f = 0
   >>> g = "0"
   >>> type(f)
   <class 'int'>
   >>> type(g)
   <class 'str'>
   >>> 

   # ERROR: variables of different types cannot be combined
   >>> print("this is a string " + 123 )
   Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   TypeError: Can't convert 'int' object to str implicitly

   # Proper way to do  this 
   >>> print("This is a string " + str(123) )
   This is a string 123
   >>> 

   # one to many var
   >>> a = b = c = 3
   >>> print(a)
   3
   >>> print(b)
   3
   >>> print(c)
   3
   >>> 
   
   # many to many
   >>> a, b, c = 1, 4, 'gerry'
   >>> print(a)
   1
   >>> print(b)
   4
   >>> print(c)
   gerry
   >>> 

   # read input
   >>> name = input("Enter your name: ")
   Enter your name: bob
   >>> print("your name is",name)
   your name is bob
   >>> 
   ################
   # Similar to get-member in pwsh
   dir(f) # see methods and properties
   type(f) # gives you the data type so you can google it to find methods and parameters

   # another way
   >>> import inspect
   >>> a = inspect.getmembers(name, predicate=inspect.isbuiltin)
   >>> type(a)
   <class 'list'>
   >>> for item in a:
   ...     print(item)
   ... 
   ('__dir__', <built-in method __dir__ of str object at 0x7faf534653018>)
   ('__format__', <built-in method __format__ of str object at 0x7faf534653018>)
   # more stuff
   >>>

   ################
   # Comment block (single line)

   '''
   multi line
   commment block
   '''

   ################
   # Data type casting
   # float to int
   >>> int(3.14159)
   3
   # string to collection
   >>> list('gerry')
   ['g', 'e', 'r', 'r', 'y']
   # int to string
   >>> chr(97)
   'a'
   # int to boolean
   >>> bool(1)
   True
   >>> bool(0)
   False
   # boolean to int
   >>> int(True)
   1
   >>> int(False)
   0
   # string to int
   >>> int('100')
   100
   # int to hex
   >>> hex(397312)
   '0x61000'
   >>>

Functions



1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79



   ################
   # Default function stuff
   
   >>> def print_args(arg1,arg2):
   ...     print(arg1,arg2)
   ... 
   >>> print_args("Hello my name is","Gerry.")
   Hello my name is Gerry.
   >>> 

   # function that returns a value
   >>> def cube(x):
   ...     return x*x*x
   ... 
   >>> cube(9)
   729
   >>>

   # function with default value for an argument
   >>> def power(num, x=1):
   ...     result = 1
   ...     for i in range(x):
   ...             result = result * num
   ...     return result
   ... 
   # here x=1(default value) and 2^1 is printed
   >>> power(2)
   2
   # here x=3 and 2^3 is printed
   >>> power(2,3)
   18
   # python can determine arguments by name
   >>> power(x=3,num=2)
   18
   >>> 

   # function with variable number of arguments
   >>> def multi_add(*args): 
   ...     result = 0
   ...     for x in args:
   ...             result = result + x
   ...     return result
   ... 
   >>> multi_add(2,4,6,7,3)
   22
   >>> 


   ################
   # Script Arguments
   # sys.argv is list of arguments passed to the script
   # sys.argv[0] is by default the path of script
   # and user defined arguments start with index 1

   # example, type `vi max.py` and paste in
   #!/usr/bin/python
   
   import sys
   
   # Print total number of arguments
   print ('Total number of arguments:', format(len(sys.argv)))
   
   # Print all arguments
   print ('Argument List:', str(sys.argv))
   
   # Print arguments one by one
   print ('First argument:',  str(sys.argv[0]))
   print ('Second argument:',  str(sys.argv[1]))
   print ('Third argument:',  str(sys.argv[2]))
   # save and exit
   # now type `chmod +x ./max.py` and `python3 ./max.py 13 23 57`
   $ python3 ./max.py 13 418 72
   Total number of arguments: 4
   Argument List: ['./max.py', '13', '418', '72']
   First argument: ./max.py
   Second argument: 13
   Third argument: 418
   $

Strings



1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125



   # mostly copied from https://ridicurious.com/20118/03/30/powershell-scripting-guide-to-python-part1/, posting here in case he removes his post for some reason

   # send text to var
   >>> hello = "this is text"
   >>> print(hello)
   this is text
   >>> 

   # check var value
   varGerry = 'gerry'
   >>> if varGerry == 'gerry':
   ...     print("hello gerry")
   ... 
   hello gerry
   >>> 

   # split method
   >>> name = 'Gerry LastName'
   >>> first, last = name.split(' ')
   >>> print(first)
   Gerry
   >>> print(last)
   LastName

   # variable expansion in strings
   >>> name = 'Gerry'
   >>> adjective = 'funny'
   >>> noun = 'person'
   >>> verb = 'runs'
   >>> print(name,'is a',adjective,noun,'that',verb)
   Gerry is a funny person that runs
   >>> 

   # Preferred:
   >>>print('{} is a {} {} that {}'.format(name, adjective,noun,verb))
   Gerry is a funny person that runs
   >>> 

   # use numbers in braces to define sequence of arguments consumed by the string
   >>> 'a:{0} b:{2} c:{1}'.format('red','green','blue')
   'a:red b:blue c:green'
   >>> 

   # adding zeros
   >>> '{0:03d}'.format(5)
   '005'
   >>>

   # using dictionaries  for formatting
   >>> coor = {'latitude': '31.24E', 'longitude': '-125.181N'}
   >>> 'Coordinates: {latitude}, {longitude}'.format(**coor)
   'Coordinates: 31.24E, -125.181N'
   >>> 

   #################
   # escape characters in string
   # represented by a backslash '\'
   # interpreted in a single and double quoted strings.
   >>> print("\\")
   \
   >>> print('\'')
   '
   >>> print("\"")
   "
   >>> print("Hello\nWorld!")
   Hello
   World!
   >>> print("Hello\tWorld!")
   Hello   World!

   # you can escape double within single but it's not required
   >>> bob = '\"bob\"'
   >>> print(bob)
   "bob"
   # same as
   >>> bob = '"bob"'
   >>> print(bob)
   "bob"
   >>> 

   ###############
   # common string operators
   >>> string = 'Gerry'
   # concatenation
   >>> string +' LastName'
   'Gerry LastName'
   # repetition
   >>> string*5
   'GerryGerryGerryGerryGerry'
   # access character at index
   >>> string[1]
   'e'
   >>> 

   # built-in string methods
   len('python') # length of string
   'python'.index('o') # find characters at an index

   # changing the case
   print('hello world'.capitalize())  # capitalize 1st char of the string
   print('hello world'.title())  # all words to title case

   # splitting a string .split()
   print(string.split(' '))  # splitting the string by white space
   print(string.split('my'))  # splitting the string by a word, eq. 'my'
   print(string.split('\n'))  # splitting by newline character

   # joining string .join()
   print(' '.join('Gerry'))  # Joining each char with a space
   print('*'.join('Gerry'))  # Joining each char with a '*'

   # reversing a string reversed()
   print(''.join(reversed('python')))  # reversing a string
   print(''.join(list('python')[::-1]))  # reversing a string / print(‘Python'[::-1]) is much more elegant and easy to remember

   # strip/trim a string .strip() , .lstrip() , .rstrip()
   print('   Hey there      '.strip())  # stripping white spaces from both ends of the string
   print('   Hey there      '.lstrip())  # stripping white spaces from left end of the string
   print('   Hey there      '.rstrip())  # stripping white spaces from right end of the string

   # string padding .rjust() .ljust(), .center()
   print('Hello'.rjust(30))
   print('Hello'.ljust(30, '-'))
   print('Hello'.rjust(30, '*'))
   print('Hello'.center(30, '_'))

Conditionals



1
2
3
4
5
6
7
8
9
10
11
12
13
14



   ## Conditionals ## 
   print("\n\nConditional:")
   x, y = 100, 100

   # conditional flow uses if, elif, else  
   if(x < y):
      st = "x is less than y"
   elif(x == y):
      st = "x is the same as y"
   else:
      st ="x is greater than y"
   print(st)

   # python doesn't have switch cases, I pretty much just use if, elif, else instead

### Loops



1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109



     ## Loops ##
   print("\n\nLoops")
   # python only has for and while loop

   # define a while loop
   x = 0
   while(x <5):
      print(x)
      x+=1  # there is no x++ operater in python

   # define a for loop
   for x in range(5,10):
      print(x)

   # use the break and continue statements
   for x in range(5,30):
      #if (x == 7): break  # when x ==7 loop will end and next block of code will execute 
      if (x %2 == 0) : continue # when x == even number, loop will go back to start and run again ignoring the even numbers and only printing odd numbers
      print(x)

   #using the enumerate() function to get index 
   days = ["Mon","Tue","Wed","Thu","Fri","Sat","Sun"]

   for i,d in enumerate(days):
      print(i,d)

   # for loop
   primes = [2, 3, 5, 7]
   for prime in primes:
      print(prime)

   # while
   # Prints out 0,1,2,3,4

   count = 0
   while count < 5:
      print(count)
      count += 1  # This is the same as count = count + 1

   # break and continue
   # Prints out 0,1,2,3,4

   count = 0
   while True:
      print(count)
      count += 1
      if count >= 5:
         break

   # Prints out only odd numbers - 1,3,5,7,9
   for x in range(10):
      # Check if x is even
      if x % 2 == 0:
         continue
      print(x)

   # else clause
   # Prints out 0,1,2,3,4 and then it prints "count value reached 5"

   count=0
   while(count<5):
      print(count)
      count +=1
   else:
      print("count value reached %d" %(count))

   # Prints out 1,2,3,4
   for i in range(1, 10):
      if(i%5==0):
         break
      print(i)
   else:
      print("this is not printed because for loop is terminated because of break but not due to fail in condition")

   ## another for loop
   #!/usr/bin/python
   # A list of shuttles 
   shuttles = ['columbia', 'endeavour', 'challenger', 'discovery', 'atlantis', 'enterprise', 'pathfinder' ]
   
   ## Read shuttles list and enumerate into index and value 
   for index, value in enumerate(shuttles):
         print index, value

   '''
   prints:
   0 columbia
   1 endeavour
   2 challenger
   3 discovery
   4 atlantis
   5 enterprise
   6 pathfinde
   '''


   ## Iterate through dictionary
   dnsservers = {}
   dnsservers = {"us":"ns1.cyberciti.com", "uk":"ns2.cyberciti.biz", "asia":"ns3.cyberciti.org"  }

   # Python for loop for key,value using dict data type
   for location, server in dnsservers.items():
      print( server, "dns server is located in" , location)

   '''
   returns:
   ns1.cyberciti.com dns server is located in us
   ns2.cyberciti.biz dns server is located in uk
   ns3.cyberciti.org dns server is located in asia
   '''

Collections and Arrays



1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157



   print("\n\nArrays")
   days = ["Mon","Tue","Wed","Thu","Fri","Sat"]
   print(days)  # this will print arrays in array format

   #printing all elements of an array
   for d in days: 
      print(d)

   ################################################################################
   # import the module, since Array is not a native data structure
   #2020-03-21-23-20-59.png

   import array
   a = array.array("I",[1,2,3,4,5]) # homogeneous, strong typed array
   type(a) # get data type of the array
   # not implicitly typecasted, you've to explicitly typecast elements with the array data type
   a = array.array("I",[1,2,3,int(4.3),5]) 
   # throws error 'TypeError: integer argument expected, got float'
   a = array.array("I",[1,2,3,4.3,5]) 

   # operations on array
   a.insert(1,7) # inserting elements
   a.pop(3) # delete and return an element
   a.reverse() # reverse the array

   # access elements
   a[0] # list first element
   a[-1] # list last element

   import array as arr

   numbers_list = [2, 5, 62, 5, 42, 52, 418, 5]
   numbers_array = arr.array('i', numbers_list)

   print(numbers_array[2:5]) # 3rd to 5th
   print(numbers_array[:-5]) # beginning to 4th
   print(numbers_array[5:])  # 6th to end
   print(numbers_array[:])   # beginning to end

   # see https://www.programiz.com/python-programming/array
   # Unless you don't really need arrays (array module may be needed to interface with C code), their use is not highly recommended.

   ##############

   # lists
   # creating empty lists
   fruits = []
   fruits = list()

   # adding elements to list
   fruits.append('apple')
   fruits.append('banana')
   fruits.append('orange')


   '''
   >>> cars.append("ford")  
   >>> print(cars)
   ['ford']
   >>> cars.append("chevy") 
   >>> print(cars)
   ['ford', 'chevy']
   >>> print(cars[0]) 
   ford
   >>> print(cars[1]) 
   chevy
   >>>print(len(cars))
   2
   '''

   # change value
   ## >>> cars[-1] = 'kia'
   ## >>> print(cars)
   ##['ford', 'kia']
   ##>>>

   # insert at element number:
   vowel.insert(3, 'u')  # .insert(index, element)

   '''
   # to reverse a list
   >>> array = list('spongebob') 
   >>> reversed(array) # doesn't work
   <list_reverseiterator object at 0x035D2790>
   >>> list(reversed(array))
   ['b', 'o', 'b', 'e', 'g', 'n', 'o', 'p', 's']
   >>>
   '''


   # assigning list elements to multiple variables
   array = [1,2,3,4,5]
   a,b,c,d,e = array
   # this will assign a the value of 1, b the value of 2 and so on...

   '''
   example
   >>> array = [2,4,6,18]
   >>> gerry, bob, tim, jim = array
   >>> gerry
   2   
   >>> time
   Traceback (most recent call last):   
   File "<stdin>", line 1, in <module>
   NameError: name 'time' is not defined
   >>> time
   Traceback (most recent call last):   
   File "<stdin>", line 1, in <module>
   NameError: name 'time' is not defined
   >>> tim
   6   
   '''

   # delete elements of an array using del, remove() and pop()
   colors = ["violet", "indigo", "blue", "green", "yellow", "orange", "red"]
   # deleting by index
   del colors[4]
   # deleting by value
   colors.remove("blue") # removes only the first occurrence
   colors.pop(3) # .pop(index) deletes and returns the element
   print(colors)

   # slicing lists
   # array[startindex:end:step]
   animal = ["dog", "cat", "cow", "pig", "giraffe"]
   print(animal[1:4]) # animal[start:end] items start through end-1
   print(animal[-3:-1])
   print(animal[2:]) # animal[start:] items start through the rest of the array
   print(animal[-4:])
   print(animal[:3]) # animal[:end] items from the beginning through end-1
   print(animal[:]) # copy of the whole array
   print(animal[1:4:2]) # animal[start:end:step]  start through not past end, by step

   # two dimensional lists

   # creating 2D lists
   array = [1, 2, [3.1, 3.2, 3.3], 4]
   array[2][1] # fetching values
   array[2][2]
   array[2][0] = 5 # assigning values
   print(array)

   # tuples cannot be modified, read only  - no add, delete
   tuple = (5, 6, 7, 18) # () parenthesis to create tuples
   print('Tuple:', tuple)
   print("Tuple index 1:", tuple[1])  # number in square bracket is index of element you want to access

   # define sets
   x = set('python')
   y = set('powershell')

   print(x - y) # All the elements in x but not in y
   # union
   print(x | y) # Unique elements in x or y or both
   # intersection
   print(x & y) # Elements in both x and y
   print(x ^ y) # Elements in x or y but not in both

Dictionaries



1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93



   #######################################################
   # hastables/dictionary
   # note a comma (,) after each key-value pair
   # and colon (:) between the Key-value pair instead of 'equals to' (=) used in PowerShell
   table = {} # empty hashtable
   table = dict()  # alternative way to create empty hastable

   # hashtable with integer keys
   table = {
      1 : 'one',
      2 : 'two',
      3 : 'three'
   }

   # hashtable is a data stucture that has key-value pair as a element
   table = {
      'firstname' : 'prateek', # in powershell single/double quotes around 'string keys' are optional, but not in python
      'lastname' : 'singh'
   }

   # adding key-value pairs or replace any existing key
   age = {}
   age['prateek']=27
   age['sam']=31
   age['susan']=25

   # get dictionary keys or values
   age.keys()
   age.values()

   # returns a boolean true/false if key matches
   age.__contains__('sam')

   # iterating through key-value pairs
   for key, value in age.items():
      "key: {0}, value: {1}".format(key, value)


   # created nested dictionary
   employee = {
      'name': {
         'firstname': 'prateek',
         'lastname': 'singh'
      },
      'dateofjoining': {
         'day': 1,
         'month': 5,
         'year': 2017
      }
   }

   age
   { 'gerry': 31, 'sam': 29, 'billy':10, 'sally': 18}
   # Deleting a key-value pair
   age.__delitem__('gerry')

   # accessing nested dictionary items
   print(table['name']['firstname'])
   print(table['dateofjoining']['year'])

   '''
   >>> age = {}                                                         
   >>> age['gerry'] = 31
   >>> age['sam'] = 29
   >>> age['billy']=10
   >>> age['sally']=18
   >>> age
   {'gerry': 31, 'sam': 29, 'billy': 10, 'sally': 18}
   >>> age.__delitem__('gerry')
   >>> age
   {'sam': 29, 'billy': 10, 'sally': 18}
   >>> age.pop('sam') # deletes, the key-value pair but returns the value
   29
   >>> age
   {'billy': 10, 'sally': 18}
   >>> age.popitem()
   ('sally', 18)
   >>> age
   {'billy': 10}
   >>>
   '''

   # sorting a dictionary
   numbers = {
      5 : 'five',
      3 : 'three',
      1 : 'one',
      2 : 'two',
      4 : 'four'
   }

   dict(sorted(numbers.items())) # ascending order
   dict(sorted(numbers.items(),reverse=True)) # descending order

Date and Time



1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39



   ######
   # date and time
   import time
   from datetime import datetime
   time.localtime(time.time()) # local time
   time.asctime(time.localtime(time.time()))
   time.gmtime(time.time()) # time in UTC
   #alternatively
   str(datetime.now())

   #import time
   time.localtime()
   print('Day:',time.localtime().tm_mday) # output: Day: 6
   print('Hour:',time.localtime().tm_hour) # output: Hour: 21
   print('Minute:',time.localtime().tm_min) # output: Minute: 13
   print('Second:',time.localtime().tm_sec) # output: Second: 11

   rightnow = time.asctime(time.localtime()) # local time
   print(rightnow) # Sat Mar 21 22:56:53 2020
   # time.gmtime(time.localtime()) # time in UTC

   # formatting date time
   from datetime import datetime
   '{:%d-%b-%Y %I:%M:%S %p}'.format(datetime.now())
   # output: '06-Apr-20118 09:39:10 PM'

   '{:%d/%m/%Y %H:%M:%S}'.format(datetime.now())
   # output: '06/04/20118 21:39:25'

   # time delta
   from datetime import datetime, timedelta

   # timedelta and future/past dates
   futuredate = datetime.now() + timedelta(days=365, hours=4, minutes=2)
   pastdate = datetime.now() + timedelta(days=-365,  hours=4, minutes=2)

   '{:%d-%b-%Y %I:%M:%S %p}'.format(futuredate)

   '{:%d-%b-%Y %I:%M:%S %p}'.format(pastdate)

Misc

  • Sleep


1
2



   import time
   time.sleep(2) # sleeps for 2 seconds
  • Do nothing


1



   pass

Comments

You May Also Enjoy