Variables and controls in Python
- 1 Data types
- 2 Variable scoping
- 3 Control statements
- 4 Short-circuiting
- 5 Homework 1
1 Data types
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
1.3 Floating-point numbers
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
- Maximum non-infinity value: Approximately
1.4 Complex numbers
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)
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""")
bool type should be
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
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
Python provides a branching control
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')
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)
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
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
0xa9f5 to a decimal number manually.
Show your work for full credits!