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Control Flow

6. Control Flow#

6.1. Basics#

For a given file with a python program, the python interpreter will start at the top and then process the file. We demonstrate this with a simple program, for example:

def f(x):
    """function that computes and returns x*x"""
    return x * x

print("Main program starts here")
print("4 * 4 = %s" % f(4))
print("In last line of program -- bye")

Main program starts here
4 * 4 = 16
In last line of program -- bye

The basic rule is that commands in a file (or function or any sequence of commands) is processed from top to bottom. If several commands are given in the same line (separated by ;), then these are processed from left to right (although it is discouraged to have multiple statements per line to maintain good readability of the code.)

In this example, the interpreter starts at the top (line 1). It finds the def keyword and remembers for the future that the function f is defined here. (It will not yet execute the function body, i.e. line 3 – this only happens when we call the function.) The interpreter can see from the indentation where the body of the function stops: the indentation in line 5 is different from that of the first line in the function body (line2), and thus the function body has ended, and execution should carry on with that line. (Empty lines do not matter for this analysis.)

In line 5 the interpreter will print the output Main program starts here. Then line 6 is executed. This contains the expression f(4) which will call the function f(x) which is defined in line 1 where x will take the value 4. [Actually x is a reference to the object 4.] The function f is then executed and computes and returns 4*4 in line 3. This value 16 is used in line 6 to replace f(4) and then the string representation %s of the object 16 is printed as part of the print command in line 6.

The interpreter then moves on to line 7 before the program ends.

We will now learn about different possibilities to direct this control flow further.

6.1.1. Conditionals#

The python values True and False are special inbuilt objects:

a = True
print(a)

True

type(a)

bool

b = False
print(b)

False

type(b)

bool

We can operate with these two logical values using boolean logic, for example the logical and operation (and):

True and True          #logical and operation

True

True and False

False

False and True

False

True and True

True

c = a and b
print(c)

False

There is also logical or (or) and the negation (not):

True or False

True

not True

False

not False

True

True and not False

True

In computer code, we often need to evaluate some expression that is either true or false (sometimes called a “predicate”). For example:

x = 30          # assign 30 to x
x > 15          # is x greater than 15

True

x > 42

False

x == 30         # is x the same as 30?

True

x == 42

False

not x == 42     # is x not the same as 42?

True

x != 42         # is x not the same as 42?

True

x > 30          # is x greater than 30?

False

x >= 30  # is x greater than or equal to 30?

True

6.2. If-then-else#

Further information

The if statement allows conditional execution of code, for example:

a = 34
if a > 0:
    print("a is positive")

a is positive

The if-statement can also have an else branch which is executed if the condition is wrong:

a = 34
if a > 0:
    print("a is positive")
else:
    print("a is non-positive (i.e. negative or zero)")

a is positive

Finally, there is the elif (read as “else if”) keyword that allows checking for several (exclusive) possibilities:

a = 17
if a == 0:
    print("a is zero")
elif a < 0:
    print("a is negative")
else:
    print("a is positive")

a is positive

6.3. For loop#

Further information

The for-loop allows to iterate over a sequence (this could be a string or a list, for example). Here is an example:

for animal in ['dog','cat','mouse']:
    print(animal, animal.upper())

dog DOG
cat CAT
mouse MOUSE

Together with the range() command (03 Data Types Structures, The Range Command), one can iterate over increasing integers:

for i in range(5,10):
    print(i)

5
6
7
8
9

6.4. While loop#

The while keyword allows to repeat an operation while a condition is true. Suppose we’d like to know for how many years we have to keep 100 pounds on a savings account to reach 200 pounds simply due to annual payment of interest at a rate of 5%. Here is a program to compute that this will take 15 years:

mymoney = 100         # in GBP
rate = 1.05           # 5% interest
years = 0
while mymoney < 200:  # repeat until 20 pounds reached
    mymoney = mymoney * rate
    years = years + 1
print('We need', years, 'years to reach', mymoney, 'pounds.')

We need 15 years to reach 207.89281794113688 pounds.

6.5. Relational operators (comparisons) in if and while statements#

The general form of if statements and while loops is the same: following the keyword if or while, there is a condition followed by a colon. In the next line, a new (and thus indented!) block of commands starts that is executed if the condition is True).

For example, the condition could be equality of two variables a1 and a2 which is expressed as a1==a2:

a1 = 42
a2 = 42
if a1 == a2:
    print("a1 and a2 are the same")

a1 and a2 are the same

Another example is to test whether a1 and a2 are not the same. For this, we have two possibilities. Option number 1 uses the inequality operator !=:

if a1 != a2:
    print("a1 and a2 are different")

Option two uses the keyword not in front of the condition:

if not a1 == a2:
    print("a1 and a2 are different")

Comparisons for “greater” (>), “smaller” (<) and “greater equal” (>=) and “smaller equal” (<=) are straightforward.

Finally, we can use the logical operators “and” and “or” to combine conditions:

if a > 10 and b > 20:
    print("A is greater than 10 and b is greater than 20")
if a > 10 or b < -5:
    print("Either a is greater than 10, or "
          "b is smaller than -5, or both.")

Either a is greater than 10, or b is smaller than -5, or both.

Use the Python prompt to experiment with these comparisons and logical expressions. For example:

T = -12.5
if T < -20:
    print("very cold")

if T < -10:
    print("quite cold")

quite cold

T < -20

False

T < -10

True

6.6. Exceptions#

Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not necessarily fatal: exceptions can be caught and dealt with within the program. Most exceptions are not handled by programs, however, and result in error messages as shown here

# NBVAL_RAISES_EXCEPTION
10 * (1/0)

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
Cell In[36], line 2
      1 # NBVAL_RAISES_EXCEPTION
----> 2 10 * (1/0)

ZeroDivisionError: division by zero

# NBVAL_RAISES_EXCEPTION
4 + spam*3

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
Cell In[37], line 2
      1 # NBVAL_RAISES_EXCEPTION
----> 2 4 + spam*3

NameError: name 'spam' is not defined

# NBVAL_SKIP
'2' + 2

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[38], line 2
      1 # NBVAL_SKIP
----> 2 '2' + 2

TypeError: can only concatenate str (not "int") to str

Schematic exception catching with all options

try:
    # code body
    pass
except ArithmeticError:
    # what to do if arithmetic error
    pass
except IndexError as the_exception:
    # the_exception refers to the exeption in this block
    pass
except:
    # what to do for ANY other exception
    pass
else:  # optional
    # what to do if no exception raised
    pass

try:
    # code body
    pass
finally:
    # what to do ALWAYS
    pass

Starting with Python 2.5, you can use the with statement to simplify the writing of code for some predefined functions, in particular the open function to open files: see https://docs.python.org/3/tutorial/errors.html#predefined-clean-up-actions.

Example: We try to open a file that does not exist, and Python will raise an exception of type FileNotFoundError because the file cannot be found:

# NBVAL_RAISES_EXCEPTION
f = open("filenamethatdoesnotexist", "r")

---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
Cell In[40], line 2
      1 # NBVAL_RAISES_EXCEPTION
----> 2 f = open("filenamethatdoesnotexist", "r")

File /opt/venv/lib/python3.11/site-packages/IPython/core/interactiveshell.py:324, in _modified_open(file, *args, **kwargs)
    317 if file in {0, 1, 2}:
    318     raise ValueError(
    319         f"IPython won't let you open fd={file} by default "
    320         "as it is likely to crash IPython. If you know what you are doing, "
    321         "you can use builtins' open."
    322     )
--> 324 return io_open(file, *args, **kwargs)

FileNotFoundError: [Errno 2] No such file or directory: 'filenamethatdoesnotexist'

If we were writing an application with a userinterface where the user has to type or select a filename, we would not want to application to stop if the file does not exist. Instead, we need to catch this exception and act accordingly (for example by informing the user that a file with this filename does not exist and ask whether they want to try another file name). Here is the skeleton for catching this exception:

try:
    f = open("filenamethatdoesnotexist","r")
except FileNotFoundError:
    print("Could not open that file")

Could not open that file

There is a lot more to be said about exceptions and their use in larger programs. Start reading Python Tutorial Chapter 8: Errors and Exceptions if you are interested.

6.6.1. Raising Exceptions#

Raising exception is also referred to as ’throwing an exception’.

Possibilities of raising an Exception

  • raise OverflowError

  • raise OverflowError, "Bath is full" (Old style, now discouraged)

  • raise OverflowError("Bath is full")

  • e = OverflowError("Bath is full"); raise e

6.6.1.1. Exception hierarchy#

The standard exceptions are organized in an inheritance hierarchy e.g. OverflowError is a subclass of ArithmeticError (not BathroomError); this can be seen when looking at help(’exceptions’) for example.

You can derive your own exceptions from any of the standard ones. It is good style to have each module define its own base exception.

6.6.2. Creating our own exceptions#

6.6.3. LBYL vs EAFP#

  • LBYL (Look Before You Leap) vs

  • EAFP (Easier to ask forgiveness than permission)

numerator = 7
denominator = 0

Example for LBYL:

if denominator == 0:
    print("Oops")
else:
    print(numerator/denominator)

Oops

Easier to Ask for Forgiveness than Permission:

try:
    print(numerator/denominator)
except ZeroDivisionError:
    print("Oops")

Oops

The Python documentation says about EAFP:

Easier to ask for forgiveness than permission. This common Python coding style assumes the existence of valid keys or attributes and catches exceptions if the assumption proves false. This clean and fast style is characterized by the presence of many try and except statements. The technique contrasts with the LBYL style common to many other languages such as C.

Source: https://docs.python.org/3/glossary.html#term-eafp

The Python documentation says about LBYL:

Look before you leap. This coding style explicitly tests for pre-conditions before making calls or lookups. This style contrasts with the EAFP approach and is characterized by the presence of many if statements.

In a multi-threaded environment, the LBYL approach can risk introducing a race condition between “the looking” and “the leaping”. For example, the code, if key in mapping: return mapping[key] can fail if another thread removes key from mapping after the test, but before the lookup. This issue can be solved with locks or by using the EAFP approach.

Source: https://docs.python.org/3/glossary.html#term-lbyl

EAFP is the Pythonic way.

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