Curso python udemy

• Integers are used to represent whole numbers:

  rank = 10
  eggs = 12
  people = 3

• Floats represent decimal numbers:

  temperature = 10.2
  rainfall = 5.98
  elevation = 1031.88

• Strings represent text:

  message = "Welcome to our online shop!"
  name = "John"
  serial = "R001991981SW"

• Lists represent arrays of values that may change during the course of the program:

  members = ["Sim Soony", "Marry Roundknee", "Jack Corridor"]
  pixel_values = [252, 251, 251, 253, 250, 248, 247]

• Dictionaries represent pairs of keys and values:

  phone_numbers = {"John Smith": "+37682929928", "Marry Simpons": "+423998200919"}
  volcano_elevations = {"Glacier Peak": 3213.9, "Rainer": 4392.1}

• Keys of a dictionary can be extracted with:

  phone_numbers.keys()

• Values of a dictionary can be extracted with:

  phone_numbers.values()

• Tuples represent arrays of values that are not to be changed during the course of the program:

  vowels = ('a', 'e', 'i', 'o', 'u')
  one_digits = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

• You can get a list of attributes of a data type has using:

  dir(str)
  dir(list)
  dir(dict)

• You can get a list of Python builtin functions using:

  dir(__builtins__)

• You can get the documentation of a Python data type using:

  help(str)
  help(str.replace)
  help(dict.values)

Sometimes you might need to convert between different data types in Python for one reason or another. That is very easy to do:

• From tuple to list:

  cool_tuple = (1, 2, 3)
  cool_list = list(cool_tuple)
  cool_list # [1, 2, 3]

• From list to tuple:

  cool_list = [1, 2, 3]
  cool_tuple = tuple(cool_list)
  cool_tuple # (1, 2, 3)

• From string to list:

  cool_string = "Hello"
  cool_list = list(cool_string)
  cool_list # ['H', 'e', 'l', 'l', 'o']

• From list to string:

  cool_list = ['H', 'e', 'l', 'l', 'o']
  cool_string = str.join("", cool_list)
  cool_string # 'Hello'

As can be seen above, converting a list into a string is more complex. Here str() is not sufficient. We need str.join(). Try running the code above again, but this time using str.join(«—», cool_list) in the second line. You will understand how str.join() works.

• Lists, strings, and tuples have a positive index system:

  ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
     0      1      2      3      4      5      6

• And they have a negative index system as well:

  ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
    -7     -6     -5     -4     -3     -2     -1

• In a list, the 2nd, 3rd, and 4th items can be accessed with:

  days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
  days[1:4]
  Output: ['Tue', 'Wed', 'Thu']

• First three items of a list:

  days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
  days[:3]
  Output:['Mon', 'Tue', 'Wed'] 

• Last three items of a list:

  days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
  days[-3:]
  Output: ['Fri', 'Sat', 'Sun']

• Everything but the last:

  days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
  days[:-1] 
  Output: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'] 

• Everything but the last two:

  days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
  days[:-2] 
  Output: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri'] 

• A dictionary value can be accessed using its corresponding dictionary key:

  phone_numbers = {"John":"+37682929928","Marry":"+423998200919"}
  phone_numbers["Marry"]
  Output: '+423998200919'

• Define functions:

  def cube_volume(a):
      return a * a * a

• Write if-else conditionals:

  message = "hello there"
   
  if "hello" in message:
      print("hi")
  else:
      print("I don't understand")

• Write if-elif-else conditionals:

  message = "hello there"
   
  if "hello" in message:
      print("hi")
  elif "hi" in message:
      print("hi")
  elif "hey" in message:
      print("hi")
  else:
      print("I don't understand")

• Use the and operator to check if both conditions are True at the same time:

  x = 1
  y = 1
   
  if x == 1 and y==1:
      print("Yes")
  else:
      print("No")

• Use the or operator to check if at least one condition is True:

  x = 1
  y = 2
   
  if x == 1 or y==2:
      print("Yes")
  else:
      print("No")

• Check if a value is of a particular type with isinstance:

  isinstance("abc", str)
  isinstance([1, 2, 3], list)
  # or directly:
   
  type("abc") == str
  type([1, 2, 3]) == lst

A for-loop is useful to repeatedly execute a block of code.

• You can create a for-loop like so:

  for letter in 'abc':
      print(letter.upper())

  output

  A
  B
  C

  • As you can see, the for-loop repeatedly converted all the items of 'abc' to uppercase.
  • The name after for (e.g. letter) is just a variable name
• You can loop over dictionary keys as follows:

  phone_numbers = {"John Smith":"+37682929928","Marry Simpons":"+423998200919"}
  for value in phone_numbers.keys():
      print(value)

  output

  John Smith
  Marry Simpsons

• You can loop over dictionary values:

  phone_numbers = {"John Smith":"+37682929928","Marry Simpons":"+423998200919"}
  for value in phone_numbers.values():
      print(value)

  output

  +37682929928
  +423998200919

• You can loop over dictionary items:

  phone_numbers = {"John Smith":"+37682929928","Marry Simpons":"+423998200919"}
  for key, value in phone_numbers.items():
      print(key, value)

  output

  John Smith +37682929928
  Marry Simpons +423998200919

• We also have while-loops. The code under a while-loop will run as long as the while-loop condition is true:

  while datetime.datetime.now() < datetime.datetime(2090, 8, 20, 19, 30, 20):
      print("It's not yet 19:30:20 of 2090.8.20")

  • The loop above will print out the string inside print() over and over again until the 20th of August, 2090.

A list comprehension is an expression that creates a list by iterating over another container.

• A basic list comprehension:

  [i*2 for i in [1, 5, 10]]

  output

  [2, 10, 20]

• List comprehension with if condition:

  [i*2 for i in [1, -2, 10] if i>0]

  output

  [2, 20]

• List comprehension with an if and else condition:

  [i*2 if i>0 else 0 for i in [1, -2, 10]]

  output

  [2, 0, 20]