Index

1. Lecture: 4-1

• Prerequisites for this lecture are: 3-1 and 3-2.

2. Python scopes

• a declaration occurs when an object is created
• by default usage is confined to the current scope

• myfunc.py

  #!/usr/bin/python3
  
  s = "hello world"  #  s is declared as a string
  i = 42             #  i is an int
  
  def func (j):
     i = 1           #  another variable, i, local
     print(j * 2)     #  to func
  
  func (2)
  print(i)

• notice how the scope changes with indentation

• $ python3 myfunc.py
  4
  42

3. Accessing global variables from a function

• myfunc2.py

  #!/usr/bin/python3
  
  s = "hello world"  #  s is declared as a string
  i = 42             #  i is an int
  
  def func (j):
     global i        #  reference global scope i
     i = 1           #  assign to global variable
     print(j * 2)
  
  func (2)
  print(i)

• a common error is to forget the global keyword in a function
  

4. Python dictionaries

• recall from our first python lecture that python has a dictionary type

• #!/usr/bin/python3
  
  d = {"spam":2, "ham":1, "eggs":10}
  
  print(d["eggs"])

• $ python3 py13.py
  10

• dictionaries can be altered
  • by adding new entries
  • by changing values
  • by removing entries
• we can test for the presence of an entry within the dictionary

• #!/usr/bin/python3
  
  d = {"spam":2, "ham":1, "eggs":10}
  print(d["eggs"])
  
  
  d["eggs"]=12
  d["bacon"]=1
  print(d["bacon"])
  print(d)

• $ python3 py14.py
  10
  1
  {"spam": 2, "ham": 1, "bacon": 1, "eggs": 12}

5. Dictionary initialisation

• we can create an empty dictionary by:

• d = {}

• we can add to a dictionary by:

• d["foobar"] = 4

6. Dictionary modification

• we can remove an entry via:

• del d["eggs"]

• we can test whether an item exists in the dictionary

• #!/usr/bin/python3
  
  d = {"spam":2, "ham":1, "eggs":10}
  
  if "eggs" in d:
      print("we have some eggs")
  else:
      print("we do not have any eggs")
  
  
  if "flour" in d:
      print("we have some flour")
  else:
      print("we do not have any flour")

Index

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