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Variables and controls in Python

Dr. Huidae Cho
Institute for Environmental and Spatial Analysis
University of North Georgia

1   Data types

1.1   Integers

The int type can handle long integers very effectively.

x = 1
type(x); print(x)
for i in range(1,100):
  x *= 10
  type(x); print(x)

Integers are basically unbounded.

Only memory limits its maximum value.

1.2   Integer representations

  • Decimal: 53
  • Binary: 0b110101
  • Octal: 0o65
  • Hexadecimal: 0x35

1.3   Floating-point numbers

The float type represents floating-point (real) numbers.

for x in (3.414, 4., .3, 1e2, 3.4e1):
  type(x); print(x)
  • Minimum non-zero value: Approximately 5e-324
  • Maximum non-infinity value: Approximately 1.79e308

1.4   Complex numbers

Use j instead of i for imaginary parts.

x = 4+5j
type(x); print(x)
y = 4-5j
z = x+y
type(z); print(z)
print(z.real); print(z.imag)

1.5   Strings

The str type stores a sequence of characters.

a = 'Hello World!'
type(a); print(a)

an_empty_string = ''
print('really'+an_empty_string+'?')

1.6   Strings: Escaping sequences

Escape sequences to include delimiters.

print("What about a single quote (')?")
print('What about a single quote (\')?')
print('What about a double quote (")?')
print("What about a single quote (\")?")
print("What about a backslash itself (\\)?")

1.7   Strings: Triple-quoted strings

Use three single/double quotes for multi-line strings.

print('''No escaping needed for '!''')
print("""Even
"multiple"
lines""")

1.8   Boolean

The bool type should be True or False, not true or false.

a = 'you'
type(a == 'you'); print(a == 'you')
if a == 'me':
  print('me')
elif a == 'you':
  print('you')
else:
  print('someone else')

1.9   Type casting

Type casting means converting one type of a variable to another.

x = '12'
type(int(x)); print(int(x))
x = '12.34'
# Oops!
type(int(x)); print(int(x))
x = 12.34
type(str(x)); print('String: ' + str(x))

2   Variable scoping

2.1   Global variables

Global variables are available “globally.”

def f():
  print(a)
a = 'Global?'
f()

Try to change a global variable.

def f():
  a = 'Local!'
  print(a)
a = 'Global?'
f()
print(a)

2.2   Global variables: Global and local

You cannot access the same variable globally and locally.

def f():
  print(a)
  a = 'Local!'
  print(a)
a = 'Global?'
f()
print(a)

Any variables that are assigned or modified inside a function become local.

2.3   Global variables: Explicit declaration

You can modify global variables inside a function.

def f():
  global a
  print(a)
  a = 'Modified global'
  print(a)
a = 'Global?'
f()
print(a)

2.4   Local variables

Local variables are only accessible from where they are defined.

def f():
  a = 'I am local!'
  print(a)
f()
print(a)

2.5   Nonlocal variables

Nonlocal variables are similar to global variables, but you cannot modify them from a nested function.

def f():
  a = 'Local'
  print('Inside f: ' + a)
  def g():
    nonlocal a
    a = 'Modified local'
    print('Inside g: ' + a)
  g()
  print('Inside f: ' + a)
a = 'Global?'
f()
print(a)

2.6   Nonlocal vs. global variables

Global variables are modifiable anywhere as long as they are declared as global.

def f():
  a = 'Local'
  print('Inside f: ' + a)
  def g():
    global a
    a = 'Modified global'
    print('Inside g: ' + a)
  g()
  print('Inside f: ' + a)
a = 'Global?'
f()
print(a)

3   Control statements

3.1   Conditionals

Python provides a branching control if, else, and elif.

a = 1
if a == 0:
  print('a is 0')
else:
  print('a is not 0')

if a == 0:
  print('a is 0')
elif a == 1:
  print('a is 1')
else:
  print('a is not 0 or 1')

3.2   Loops: for

A for loop takes an array-like data type.

# tuple
for i in (1, 2, 3):
  print(i)

# list
for i in [1, 2, 3]:
  print(i)

3.3   Loops: while

A while loop tests a condition and loops the block as long as the condition is true.

# prints 1 through 10
a = 1
while a <= 10:
  print(a)
  a = a+1

4   Short-circuiting

Short-circuiting means taking a shortcut in a conditional statement.

def f():
  return x == 1
def g():
  global x
  x += 1
  return x == 3
x = 1
print('Initial: ' + str(x))
if f() or g():
  print('f True, g False: ' + str(x))
x = 2
if f() or g():
  print('f False, g True: ' + str(x))

5   Homework 1

Convert 0xa9f5 to a decimal number manually.

Show your work for full credits!