Python I

Question 1

attribute for getting the dimensions of a numpy array (e. g. (3, 4, 2))

Answer 1

arr.shape

Question 2

attribute for getting the datatype of a numpy array

Answer 2

arr.dtype

Question 3

attribute for getting the number of dimensions of a numpy array

Answer 3

arr.ndim

Question 4

attribute for getting the number of all elements of a numpy array

Answer 4

arr.size

Question 5

creating an array in numpy

Answer 5

np.array(my_list)

Question 6

creating a range in numpy

Answer 6

np.arange(start, stop, step)

Question 7

creating ranges of values in numpy

Answer 7

np.linspace(start, stop, numberOfElems, endpoint=True/False)

Question 8

get multiple elements of a numpy array (index)

Answer 8

one_dim[[3, 4, 6, 15]] (gets elements at index 3, 4, 5, 6 and 15)

Question 9

get the element of the 3rd row and 4th column of a numpy array

Answer 9

arr[2, 3] or arr[(2, 3)]

Question 10

get the second row of a multidimensional numpy array

Answer 10

arr[1]

Question 11

get the second column of a multidimensional numpy array

Answer 11

arr[: , 1]

Question 12

extract the bottom right 2x2 corner of a 3x4 numpy array

Answer 12

arr[1:3, 2:4]

Question 13

indexing of numpy arrays is also possible via masks

Answer 13

mask = np.array([[True, False, False, True],
[False, True, False, True],
[False, False, True, True]])
print(“two_dim[mask]”)
print(two_dim[mask])

(Returns flat (1D) array of elements where mask = True)

Question 14

only extract even number out of a numpy array

Answer 14

mask = (arr % 2) == 0
print(arr[mask])

Question 15

additional information numpy arrays

Answer 15

x2 = two_dim[:][2]

This will first create a slice of the array (in this case, this is simply the entire array again) and then access index 2 in that sliced array, which corresponds to “two_dim[2]”

Question 16

additional information numpy arrays

Answer 16

x2 = two_dim[:][2]

This will first create a slice of the array (in this case, this is simply the entire array again) and then access index 2 in that sliced array, which corresponds to “two_dim[2]”

Question 17

additional information numpy arrays

Answer 17

x2 = two_dim[:][2]

This will first create a slice of the array (in this case, this is simply the entire array again) and then access index 2 in that sliced array, which corresponds to “two_dim[2]”

Question 17

additional information numpy arrays

Answer 17

x2 = two_dim[:][2]

This will first create a slice of the array (in this case, this is simply the entire array again) and then access index 2 in that sliced array, which corresponds to “two_dim[2]”

Question 18

set all elements from index 2 to 4 to zero in a numpy array

Answer 18

one_dim[2:5] = [0, 0, 0]

or

one_dim[2:5] = 0 (broadcasting)

Question 19

set all entries of a twodimensional numpy array to 0

Answer 19

two_dim[:] = 0

Question 20

set the second to third element of the second row of a numpy array to 0

Answer 20

two_dim[1, 1:4] = 0

(also subarrays can be broadcasted, e. g.
two_dim[:, 1:3] = [7, 8])

Question 21

When can numpy arrays be reshaped?

Answer 21

When the number of elements stays the same.

a = np.arange(24)
a = a.reshape(2, 3, 4)
a = a.reshape(4, 6)
a = a.reshape(6, -1)

Question 22

add an empty dimension

Answer 22

a1 = a[:, None, :]
a2 = a[:, np.newaxis, :]

Question 23

Other important numpy functions

Answer 23

print(np.append(a, np.array([1, 2, 3])))
print(np.concatenate([a, a, a]))
print(np.repeat(a, repeats=5))
print(np.tile(a, reps=5))

Question 24

important: sum in numpy arrays

Answer 24

Sum (similar functions available for: mean, standard deviation, etc.) arr = np.arange(2 * 3 * 4).reshape((2, 3, 4)) * 3 print(arr.sum(axis=0)) # Sum along first axis/dimension (axis 0 will be "collapsed" -> shape: (3, 4)) print(arr.sum(axis=1)) # Sum along second axis/dimension (axis 1 will be "collapsed" -> shape: (2, 4)) print(arr.sum(axis=2)) # Sum along second axis/dimension (axis 2 will be "collapsed" -> shape: (2, 3)) print(arr.sum(axis=(1, 2))) # Sum along second + third axis (axis 1 + 2 will be "collapsed" -> shape: (2,)) print(arr.sum()) # "Global" sum along all axes/dimensions

Question 25

Get the (flat!) index of the minimum of a numpy array and transform it into an array-shaped index

Answer 25

i_min_flat = arr.argmin() i_min_shape = np.unravel_index(i_min_flat, shape=arr.shape)

Question 26

numpy functions with conditionals

Answer 26

print(np.where(arr % 2 == 0, arr, other)) # If condition is True, take from "arr", else take from "other" print((arr > 50).all()) # Check if all elements meet the condition print((arr > 50).any()) # Check if at least one element meets the condition

Question 27

random values in numpy

Answer 27

rng = np.random.default_rng(seed=1234) arr = rng.random((4, 3)) arr = rng.integers(low=1, high=25, size=5) arr = rng.choice(np.arange(100), size=10, replace=False)

Question 28

sort a numpy array

Answer 28

np.sort(arr) or arr.sort() - in-place

Question 29

important info for numpy slicing and reshaping

Answer 29

Reshaping and slicing does not copy the original array, it only creates another "view" on the data.

Question 30

regex - search for the first occurence of a pattern

Answer 30

re.search(pattern, text)

Question 31

regex - find a pattern at the beginning of the string

Answer 31

re.match(pattern, text)

Question 32

regex - groups

Answer 32

Groups can be used to only search for subpatterns within a search pattern (e. g. "H(all)o, how (are) you?") The found patterns can be accessed like this (0 is the whole pattern, 1 is the first group) match_object.group(1)

Question 33

Getting additional information from regex group match objects

Answer 33

match_object.start(1) match_object.end(1) match_object.span(1)

Question 34

regex - find all occurences of a pattern

Answer 34

re.findall(pattern, text) -> list (re.finditer does the same, but does it one item at a time and returns a match object)

Question 35

regex - a set of characters to match

Answer 35

[...] e. g. "[cbr]at"

Question 36

regex - specify ranges

Answer 36

[...] e. g. "[0-5]"

Question 37

regex - negate patterns

Answer 37

^ e. g. "[^0-9]"

Question 38

regex - predefined group for digits

Answer 38

\d

Question 39

regex - predefined group for non-digits

Answer 39

\D

Question 40

regex - predefined group for whitespace characters

Answer 40

\s

Question 41

regex - predefined group for non-whitespace characters

Answer 41

\S

Question 42

regex - predefined group for word characters

Answer 42

\w

Question 43

regex - predefined group for non-word characters

Answer 43

\W

Question 44

regex - match any character (except newline)

Answer 44

. e. g. ".m."

Question 45

regex - search for alternative patterns

Answer 45

| e. g. "[bcr]at|dog"

Question 46

regex - match any number of repetitions (also 0)

Answer 46

* e. g. "([bcr]at|dog)*"

Question 47

regex - match at least one repetition

Answer 47

+ e. g. "([bcr]at|dog)+"

Question 48

regex - be not- greedy (not search for as many repetitions as possible)

Answer 48

? (suffix)

Question 49

regex - positive lookahead

Answer 49

only match if the following is XYZ "(?=XYZ)"

Question 50

regex - negative lookahead

Answer 50

only match if it is not followed by XYZ "(?!XYZ)"

Question 51

regex - positive lookbehind

Answer 51

only match if the preceeding is XYZ "(?<=XYZ)"

Question 52

regex - negative lookbehind

Answer 52

only match if the preceeding is not XYZ "(?

Question 53

call a program with the subprocess module

Answer 53

p = subprocess.Popen(["some_program", "argument1", "argument2"]) - will not wait for the program to finish p.wait(timeout=2) - will wait 2 seconds for the program to finish (except subprocess.TimeoutExpired)

Question 54

get the output and errors of a program

Answer 54

p = subprocess.Popen(["ping", "www.jku.at"], stdout=subprocess.PIPE, stderr=subprocess.PIPE) outs, errs = p.communicate(timeout=15)

Question 55

simple way of calling a program

Answer 55

result = subprocess.run(["some_program", "argument1", "argument2"]) result.returncode result.stdout result.stderr

Question 56

only execute code in the main process

Answer 56

if __name__ == "__main__"

Question 57

use multiprocessing

Answer 57

from multiprocessing import Pool arguments = list(range(100)) without dynamic iterable: with Pool(4) as p: pool_returns = p.map(fcn, arguments) with dynamic iterable: with Pool(4) as p: pool_returns = p.imap(fcn, arguments) (or imap_unordered) for multiple arguments, starmap must be used

Question 58

add a command line argument

Answer 58

parser = argparse.ArgumentParser() parser.add_argument("-n3", "--int_number3", type=int, required=True, help="required int")

Question 59

add a command line argument which accepts mutiple inputs

Answer 59

parser.add_argument("--numbers", nargs=3, type=int) parser.add_argument("--items", nargs="+") parser.add_argument("--items", nargs="*")

Question 60

access command line arguments

Answer 60

args = parser.parse_args() my_filename = args.filename

Question 61

fall back to the default implementation of "==" if the if-statement returns False

Answer 61

def __eq__(self, other): if isinstance(other, Point): return self.x == other.x and self.y == other.y return NotImplemented

Question 62

override output of the line "p1"

Answer 62

__repr__(self)

Question 63

override output of the line "print(p1)"

Answer 63

__str__(self)

Question 64

override indexing

Answer 64

__getitem__(self, key)

Question 65

override "+"

Answer 65

__add__(self, other)

Question 66

get the type of an object

Answer 66

type(dog1).__name__

Question 67

check if a variable is either an integer of a floating point number

Answer 67

isinstance(num, (int, float))

Question 68

syntax of a static method

Answer 68

@classmethod def some_class_method(cls): print(f"this is a class method: {cls}")

Question 69

read file content

Answer 69

with open("my_module.py", "r") as f: print(f.read()) with open("my_module.py") as f: for line in f: print(line, end="") with open("my_module.py") as f: while chars := f.read(10): print("Chunk:") print(chars)

Question 70

write file content

Answer 70

with open("some_file.txt", "w") as f: f.write("This overwrites an existing file\nor creates a new one with this text!\n") with open("some_file.txt", "a") as f: f.write("And here this line is appended.\n") with open("some_file.txt", "a") as f: print("And another line via print()!", file=f)

Question 71

get current working directory

Answer 71

os.getcwd()

Question 72

create a directory

Answer 72

os.makedirs("new_directory", exist_ok=True)

Question 73

rename files or directories

Answer 73

os.rename("new_directory", "my_new_new_directory")

Question 74

remove a directory

Answer 74

os.removedirs("my_new_new_directory")

Question 75

remove a file

Answer 75

os.remove("some_file.txt")

Question 76

list all contents of a directory

Answer 76

os.listdir()

Question 77

os.path methods

Answer 77

path = os.path.join("some", "directory", "filename.py") head, tail = os.path.split(path) os.path.dirname(path) os.path.basename(path) root, ext = os.path.splitext(path) - extensions of a file os.path.exists(path)

Question 78

check if a path is a file

Answer 78

os.path.isfile(path))

Question 79

check if a path is a directory

Answer 79

os.path.isdir(".") - "." refers to the current working directory

Question 80

os.path methods

Answer 80

path = os.path.join("some", "directory", "filename.py") head, tail = os.path.split(path) os.path.dirname(path) os.path.basename(path) root, ext = os.path.splitext(path) - extensions of a file os.path.exists(path) os.path.abspath(path)

Question 81

list of all files and subdirectories in the current working directory

Answer 81

glob.glob("*")

Question 82

search the current working directory for python files

Answer 82

glob.glob("*.py")

Question 83

search for files and folders recursively

Answer 83

glob.glob("**", recursive=True)

Question 84

search in a directory and all subdirectories for python files

Answer 84

glob.glob(os.path.join("some_folder", "**", "*.py"), recursive=True)

Question 85

pickle

Answer 85

import dill as pickle always use with("path", "mode") as f pickle.dump(my_objects, f) data = pickle.load(f)

Question 86

import modules

Answer 86

import sys import sys as system from os import path import os, sys from os import path, makedirs from my_module import add as my_add

Question 87

create a list

Answer 87

empty_list = [] empty_list = list() a_list = [1, 2.3456, 3, "abc"]

Question 88

navigating through nested lists

Answer 88

nested_list_element = nested_list[2][3][0]

Question 89

check if an element is contained in a list

Answer 89

"abc" in a_list

Question 90

sort lists

Answer 90

numeric_list.sort() - in-place sorted_list = sorted(numeric_list) - numeric_list is not changed

Question 91

create a tuple

Answer 91

empty_tuple = () empty_tuple = tuple() single_element_tuple = (1,) a_tuple = 1, "2", 3 a_tuple = (1, "2", 3)

Question 92

create a set

Answer 92

empty_set = set() my_set = {1, 2, 3, 3}

Question 93

add/remove elements to a set

Answer 93

my_set.add(4) my_set.remove(2)

Question 94

common operations on sets

Answer 94

Union new_set = my_set | {1, 4, 7} new_set = my_set.union({1, 4, 7}) Intersection new_set = my_set & {1, 4, 7} new_set = my_set.intersection({1, 4, 7}) Difference new_set = my_set - {1, 4, 7} new_set = my_set.difference({1, 4, 7})

Question 95

create a dictionary

Answer 95

empty_dict = {} empty_dict = dict() some_key = "abc" dictionary = dict(some_key=3.24, other_key="twh") dictionary2 = {"stringkey": 55, some_key: "someitem", 23: 4}

Question 96

get all keys or values of a dictionary

Answer 96

dictionary.keys() dictionary.values() dictionary.items() - tuples

Question 97

additional dictionary stuff

Answer 97

Remove key and return its associated value removed = dictionary2.pop("stringkey") Get a value and use a default value if key does not exist value = dictionary.get("nonexistingkey", "defaultvalue")

Question 98

dictionary iteration

Answer 98

for key in dictionary: (Equal to: dictionary.keys()) print(key) for value in dictionary.values(): print(value) for key_value in dictionary.items(): print(key_value) for key, value in dictionary.items(): print(key, value)

Question 99

list comprehensions

Answer 99

old_list = [1, 2, 3, 4, 5] new_list = [str(i) for i in old_list] new_list = [str(i) for i in old_list if i < 3] new_list = [str(i) if i < 5 else "end" for i in old_list]

Question 100

reverse a list

Answer 100

reversed_list = some_list[::-1]

Question 101

unpacking

Answer 101

a, b, c, d = [1, 2, 3, 4] a, b, *rest = [1, 2, 3, 4] a, *rest, d = [1, 2, 3, 4]

Question 102

for loop with index

Answer 102

for i, elem in enumerate(a_tuple): print(i, elem)

Question 103

iterate through a string with index

Answer 103

for i, char in enumerate("hello"): print(f"loop iteration: {i}, element: '{char}'")

Question 104

integer division

Answer 104

// floor value of a division

Question 105

string functions

Answer 105

uppercase_string = conc_strings.upper() lowercase_string = "SILENCE!".lower() number_of_bla = repeated_string.count("bla") separated_elements = "my-string-elements".split("-") joined_elements = ";".join(separated_elements) replacement = "This is a placeholder".replace("placeholder", "string") check_for_substring = "string" in conc_strings locate_substring = replacement.find("is a")

Question 106

formatted strings

Answer 106

formatted_string = "You can format floating point values as :width.precision " \ f"{long_number:10.5f}" another_formatted_string = f"{long_number:.{n_decimals}f}"

Question 107

console

Answer 107

print("Hello, world!") my_input = input("Please enter something: ")