Elements of Programs

Names (variables, functions, modules, etc)

Variables, functions, modules, classes, etc, etc are all a class of text called indentifiers. They are names for values. A value can be a simple as a number, or a string of characters. It can also be something more esoteric as a function name.

Identifier rules for Python:

  • The first letter of an identifier must start with either an _ (underscore) or any letter (e.g. _blah).
  • Identifiers allow numbers after the first character (e.g. foo9)
  • Identifiers cannot contain symbols, e.g. $ or #
  • Identifiers are case sensitive.
In [1]:
Spam = 22
spam = 23
In [2]:
print(Spam, spam)
22 23

If you attempt to access an indentifier (like a variable or function name) before it is defined, you will raise a NameError "exception". It is how Python crashes your program in the case of an unhandled error.

In [3]:
print(x)
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-3-fc17d851ef81> in <module>()
----> 1 print(x)

NameError: name 'x' is not defined
In [4]:
x = 10
In [5]:
print(x)
10

Keywords, aka "Reserved Words"

Python, as a language, must have a vocabulary. Words like def, for, in, else, etc (see slides for link to entire list), are 'reserved'. They cannot be used as identifiers, such as the failure below as I attempt to assign to a variable named def, which results in a SyntaxError.

In [6]:
def = 10
print(def)
  File "<ipython-input-6-10c66231acbb>", line 1
    def = 10
        ^
SyntaxError: invalid syntax

However, how can I assign to print!? This is slight but fundamental difference. The "built-in" functions, like print, min, max, list, etc are not reserved words. They are pre-defined identifiers! THey are included automatically for your convenience. Functions names, being identifiers, can be re-assigned willy-nilly. You could overwrite the print() function to become a variable name holding the integer 10! Notice how print() no longer works after that reassignment.

In [7]:
print = 10
In [8]:
print("Hello World")
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-8-fb1c7aebe012> in <module>()
----> 1 print("Hello World")

TypeError: 'int' object is not callable

Literals

Literals are values that are exactly as written. If you see a 10, 3.14, or "foo" they represent those values as written! They are literally the values they represent. In the example below, we show how value can be assigned a literal value of 20. Then, the print() function prints the identifier and the literal 25. Take note: an identifier does not necessarily fully describe the value it is representing. Do you know what data value points to if I set it somewhere else in the code that you did not see? No. You'd have to ask the name to evaluate itself into the value it represents. A literal needs no such work, you know what it is immediately.

In [1]:
value = 20
In [2]:
print(value, 25)
20 25

Expressions and Statements

In [4]:
a,b,c = eval(input("Enter three numbers (a,b,c): "))

print(a,b,c)

eval("3,67,12")

"foo"

22

print("Hello", a, b, c, 140, "World")

2**3

print("Hello", "World")

print()
Enter three numbers (a,b,c): 34, 6543,23
34 6543 23
Hello 34 6543 23 140 World
Hello World

Reusing variable names

In [6]:
counter = 0
counter = counter + 1
counter = counter + 1
counter
Out[6]:
2

Simulatenous Assignment

One great Python feature is setting multiple variable names to values simultaneously. Take x and y as set immediately below:

In [7]:
x = 764
y = 535

Now, we can calculate the difference and sum of those two values "simultaneously", that is we do it in one simple line.

In [8]:
_sum, diff = x+y, x-y
print(_sum, diff)
1299 229

The eval() Function

You will learn that eval is convenient, but I will tell you it's a dangerous function to use beyond academic exercises. For now, it'll do the job. We leverage eval to pull in multiple values when entered at the keyboard (a string of numbers separated by commas).

In [9]:
n1, n2, n3, n4 = eval("12,345,3,54")
print(n1, n2, n3, n4)
12 345 3 54

Swapping Values

A not-to-uncommon action is to swap values between two (or, more) variables. To do that, there is the "traditional" method and the "Pythonic" method.

Let's start by defining two values, x and y.

In [10]:
x = 75678
y = 65456

The "traditional" method of swapping two values is to use a "temporary" variable to hold a value you need. This variable is needed just to hold and then retrieve that value.

In [11]:
temp = x
x = y
y = temp
print(x, y)
65456 75678

The "pythonic" method (aka "The Python Way of Doing Things

Definite loops (for loops)

Loops, such as the for loop, is a type of code repetition that is controlled by counting items from a collection of items. More directly, a for loop can use a list of numbers, for example, to count how many times a loop repeats. The loop below repeats four times because there are four items in the list [1,2,3,4]. Other types of sequences are strings, files, tuples, dictionaries, etc.

Additionally, not only do loops count sequences to control how many times it repeats it can also give you the value of the item that is counted next in the sequence it is working with. We do this by way of a "loop index variable". It is a necessity to have at least one loop index variable defined (e.g. i, below). When the loop runs, the variable i is set to 1, 2, 3, 4 in that order. You can then do something with this data, such as printing its value.

In [12]:
# Prints 1,2,3,4
for i in [1,2,3,4]:
    print(i)
1
2
3
4
In [13]:
# Prints 1,3,5,7,9
for i in [1,3,5,7,9]:
    print(i)
1
3
5
7
9

The range() function

Immediately, you'll notice that using lists to control your loops isn't 'scalable'. What if you wanted to loop 1000 times? You'd end up writing a list holding 1000 numbers. It visually, practically and logically doesn't make sense!

So, there is a special function called range() whose purpose to to create a "virtual" sequence of numbers by just specifing things like the start, stop and step values of a sequence! It's very compact and helpful.

When printed/evaluated a "range object" doesn't tell you much.

In [14]:
range(1000)
Out[14]:
range(0, 1000)

Force a range object into a Python list. Now you can see the range of numbers in its full glory.

In [15]:
print(list(range(1000)))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493, 494, 495, 496, 497, 498, 499, 500, 501, 502, 503, 504, 505, 506, 507, 508, 509, 510, 511, 512, 513, 514, 515, 516, 517, 518, 519, 520, 521, 522, 523, 524, 525, 526, 527, 528, 529, 530, 531, 532, 533, 534, 535, 536, 537, 538, 539, 540, 541, 542, 543, 544, 545, 546, 547, 548, 549, 550, 551, 552, 553, 554, 555, 556, 557, 558, 559, 560, 561, 562, 563, 564, 565, 566, 567, 568, 569, 570, 571, 572, 573, 574, 575, 576, 577, 578, 579, 580, 581, 582, 583, 584, 585, 586, 587, 588, 589, 590, 591, 592, 593, 594, 595, 596, 597, 598, 599, 600, 601, 602, 603, 604, 605, 606, 607, 608, 609, 610, 611, 612, 613, 614, 615, 616, 617, 618, 619, 620, 621, 622, 623, 624, 625, 626, 627, 628, 629, 630, 631, 632, 633, 634, 635, 636, 637, 638, 639, 640, 641, 642, 643, 644, 645, 646, 647, 648, 649, 650, 651, 652, 653, 654, 655, 656, 657, 658, 659, 660, 661, 662, 663, 664, 665, 666, 667, 668, 669, 670, 671, 672, 673, 674, 675, 676, 677, 678, 679, 680, 681, 682, 683, 684, 685, 686, 687, 688, 689, 690, 691, 692, 693, 694, 695, 696, 697, 698, 699, 700, 701, 702, 703, 704, 705, 706, 707, 708, 709, 710, 711, 712, 713, 714, 715, 716, 717, 718, 719, 720, 721, 722, 723, 724, 725, 726, 727, 728, 729, 730, 731, 732, 733, 734, 735, 736, 737, 738, 739, 740, 741, 742, 743, 744, 745, 746, 747, 748, 749, 750, 751, 752, 753, 754, 755, 756, 757, 758, 759, 760, 761, 762, 763, 764, 765, 766, 767, 768, 769, 770, 771, 772, 773, 774, 775, 776, 777, 778, 779, 780, 781, 782, 783, 784, 785, 786, 787, 788, 789, 790, 791, 792, 793, 794, 795, 796, 797, 798, 799, 800, 801, 802, 803, 804, 805, 806, 807, 808, 809, 810, 811, 812, 813, 814, 815, 816, 817, 818, 819, 820, 821, 822, 823, 824, 825, 826, 827, 828, 829, 830, 831, 832, 833, 834, 835, 836, 837, 838, 839, 840, 841, 842, 843, 844, 845, 846, 847, 848, 849, 850, 851, 852, 853, 854, 855, 856, 857, 858, 859, 860, 861, 862, 863, 864, 865, 866, 867, 868, 869, 870, 871, 872, 873, 874, 875, 876, 877, 878, 879, 880, 881, 882, 883, 884, 885, 886, 887, 888, 889, 890, 891, 892, 893, 894, 895, 896, 897, 898, 899, 900, 901, 902, 903, 904, 905, 906, 907, 908, 909, 910, 911, 912, 913, 914, 915, 916, 917, 918, 919, 920, 921, 922, 923, 924, 925, 926, 927, 928, 929, 930, 931, 932, 933, 934, 935, 936, 937, 938, 939, 940, 941, 942, 943, 944, 945, 946, 947, 948, 949, 950, 951, 952, 953, 954, 955, 956, 957, 958, 959, 960, 961, 962, 963, 964, 965, 966, 967, 968, 969, 970, 971, 972, 973, 974, 975, 976, 977, 978, 979, 980, 981, 982, 983, 984, 985, 986, 987, 988, 989, 990, 991, 992, 993, 994, 995, 996, 997, 998, 999]

A simple for loop that runs 1000 times from 0..999 (for a total of 1000 numbers). Ranges, by default always start enumerating at zero.

In [16]:
for i in range(1000):
    print(i)
0
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