less = [] greater = [] for x in [1, 3, 4, 2, 8, 6]: if x < 3: less.append(x) else: greater.append(x) print("less =", less, "greater =", greater)

less = [] greater = [] for y in [22, 10.3, 9, 6, 44]: if y < 20: less.append(y) else: greater.append(y) print("less than 20 =", less, "greater than 20 =", greater)

Variables and Arguments Passing

When assigning a variable (or name) in Python, you are creating a reference to the object shown on the righthand side of the equals sign. Consider a list of integers.

a = [1, 2, 3] a

b = a b

In some languages, the assignment of b will cause the data [1, 2, 3] to be copied. In Python, a and b actually now refer to the same object, the original list [1, 2, 3]

Conditionals

Python has several built-in keywords for conditional logic, loops, and other standard control flow concepts found in other programming languages.

if, elif, else

The if statements is one of the most well-known control flow statement types. It checks a condition that, if True, evaluates the code in the block that follows.

x = -5 if x < 0: print("It's negative")

and if the statement can be optionally followed by one or more elif blocks and a catchall else block if all of the conditions are False

print(y) print(x) if int(x) < 0: print("It's negative") elif int(x) == 0: print("Equal to 0") elif 0 < int(x) < 5: print("Positive but smaller than 5") else: print("Positive and larger than or equal to 5")

For Loops

For loops are for iterating over a collection (like a list or tuple) or an iterater. The standard syntax for a loop is:

for value in collection:

do something with value

sequence = [1, 2, 0, 4, 6, 5, 2 ,1] total = 0 for value in sequence: total += value # total = total + value print("total =", total)

You can advance a for loop to the next iteration, skipping the remainder of the block, using the continue keyword. Consider this code, which sums up integers in a list and skips None values

sequence = [1, 2, None, 4, None, 5] total = 0 for value in sequence: if value is None: continue total += value print(total)

A for loop can be exited altogether with the break keyword. This code sums elements of the list until 5 is reached.

sequence = [1, 2, 0, 4, 6, 5, 2, 1] total_until_5 = 0 for value in sequence: if value == 5: break total_until_5 += value print(total_until_5)

The break keyword only terminates the innermost for loop; any outer for loops will continue to run

for i in range(4): for j in range(4): if j > 1: break print((i, j))

While Loop

A while loop specifies a condition and a block of code that is to be executed until the condition evaluates to False or the loop is explicitly ended with break

x = 256 total = 0 while x > 0: print(x) if total > 500: break total += x print (total) x //= 2

import random z = random.randint(1,10) x = 0 input_a = 1 while z != input_a: x += 1 input_a = int(input("Enter your guess...")) if int(input_a) == z: print("correct... the number was", end = ' ') print(z) print("It took you", end = ' ') print(x, end = ' ') print("tries to guess the number.") if int(input_a) < z: print("too low") if int(input_a) > z: print("too high")

pass is the “no-op” (or "do nothing") statement in Python. It can be used in blocks where no action is to be taken (or as a placeholder for code not yet implemented); it is required only because Python uses whitespace to delimit blocks

x = 0 if x < 0: print("negative!") elif x == 0: print("zero") pass else: print("positive!")

Range

seq = [10, 20, 30, 40] for i in range(len(seq)): print("element %d: %d" %(i, seq[i]))

%formatting %d - integer %g - scientific %f - floating number %0.2f - 2 decimal places %s - string

Built in Sequences

Python has a handful of useful sequence functions that you should familiarize yourself with and use at any opportunity.

enumerate

It’s common when iterating over a sequence to want to keep track of the index of the current item.

# enumerate function l1 = ["eat", "sleep", "repeat"] # creating enumerate objects obj1 = enumerate(l1) print("Return type:", type(obj1)) print(list(enumerate(l1)))

sorted

The sorted function returns a new sorted list from the elements of any sequence.

a = [7, 1, 2, 6, 0, 3, 2] sorted(a)

print(sorted("COP2000 Python Programming"))

zip

zip “pairs” up the elements of a number of lists, tuples, or other sequences to create a list of tuples.

seq1 = ["Uno", "Dos", "Tres"] seq2 = ["One", "Two", "Three"] zipped = zip(seq1, seq2) list(zipped)

zip can take an arbitrary number of sequences, and the number of elements it produces is determined by the shortest sequence.

seq3 = [False, True] list(zip(seq1, seq2, seq3))

A common use of zip is simultaneously iterating over multiple sequences, possibly also combined with enumerate.

for index, (a,b) in enumerate(zip(seq1, seq2)): print(f"{index}: {a}, {b}") # https://realpython.com/python-f-strings/

reversed

reversed iterates over the elements of a sequence in reverse order

list(reversed(range(10)))

Lambda Functon

Python has support for so-called anonymous or lambda functions, which are a way of writing functions consisting of a single statement, the result of which is the return value. They are defined with the lambda keyword, which has no meaning other than “we are declaring an anonymous function”

def short_function(x): return x * 2

can be written as

lambda x: x *2

def apply_to_list(some_list, f): return [f(x) for x in some_list] ints = [2, 4, 6, 8, 1, 3, 5] apply_to_list(ints, lambda x: x ** 2)

Files and OS

To open a file for reading or writing, use the built-in open function with either a relative or absolute file path and an optional file encoding.

! wget "https://www.gutenberg.org/files/76/76-0.txt" -0 adventures.txt ! (head -10 adventures.txt) > adventures10.txt

f = open("adventures10.txt", encoding = "utf-8") for linne in f: print(line)

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