lec-08-notes.txt

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SLIDES: naming, context, frames
==================================================

PythonTutor, getting used to looking at frames:

def f():
    x = 1
    print(x)

def g():
    y = 2
    print(y)

def h():
    z = 3
    print(z)

f()
g()
h()

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def f():
    x = 1
    print(x)
    g()

def g():
    y = 2
    print(y)
    h()

def h():
    z = 3
    print(z)

f()

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def f():
    x = 1
    print(x)
    g()

def g():
    x = 2
    print(x)
    h()

def h():
    x = 3
    print(x)

f()

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LESSONS ABOUT LOCAL VARIABLES
==================================================

# function definition vs invocation

def set_x():
    x = 100

print(x)

# LESSON 1: functions don't execute unless they're called

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# variable lifetime

def set_x():
    x = 100

set_x()
print(x)

# LESSON 2: variables created in a function die after function returns

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# function variables do not persist between calls

def count():
    x = 1
    x += 1
    print(x)

count()
count()
count()

# LESSON 3: variables start fresh every time a function is called again

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# why doesn't this work?

def display_x():
    print(x)

def main():
    x = 100
    display_x()

main()

# LESSON 4: you can't see the variables of other function invocations,
  even those that call you

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# LESSONS ABOUT GLOBAL VARIABLES
==================================================

VERSION 1:

# show global frame in PythonTutor

msg = 'hello' # global, outside any func

def greeting():
    print(msg)

print('before: ' + msg)
greeting()
print('after: ' + msg)

# LESSON 5: you can generally just use global variables inside a function

----------------------------------------

VERSION 2:

msg = 'hello'

def greeting():
    msg = 'welcome!'
    print('greeting: ' + msg)

print('before: ' + msg)
greeting()
print('after: ' + msg)

# LESSON 6: if you do an assignment to a variable in a function, Python assumes you want it local

----------------------------------------

VERSION 3:

msg = 'hello'

def greeting():
    print('greeting: ' + msg)
    msg = 'welcome!'

print('before: ' + msg)
greeting()
print('after: ' + msg)

# LESSON 7: assignment to a variable should be before its use in a
# function, even if there's a a global variable with the same name

VERSION 4:

msg = 'hello'

def greeting():
    global msg
    print('greeting: ' + msg)
    msg = 'welcome!'

print('before: ' + msg)
greeting()
print('after: ' + msg)

# LESSON 8: use a global declaration to prevent Python from creating a
# local variable when you want a global variable

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# LESSONS ABOUT ARGUMENT PASSING

def f(x):
    x = 'B'
    print('inside: ' + x)

val = 'A'
print('before: ' + val)
f(val)
print('after: ' + val)

# LESSON 9: in Python, arguments are "passed by value", meaning
# reassignments to a parameter don't change the argument outside


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x = 'A'

def f(x):
    x = 'B'
    print('inside: ' + x)

print('before: ' + x)
f(x)
print('after: ' + x)

# LESSON 10: it's irrelevant whether the argument (outside) and parameter
# (inside) have the same variable name