This tutorial will discuss how the python interpreter shares the processing among the source code. To prioritize the control python used below keywords to direct the control flow.<\/p>\n\n\n\n
if<\/code>\u00a0Statements<\/li>- 2.\u00a0
for<\/code>\u00a0Statements<\/li>- 3. The\u00a0
range()<\/code>\u00a0Function<\/li>- 4.\u00a0
break<\/code>\u00a0and\u00a0continue<\/code>\u00a0Statements, and\u00a0else<\/code>\u00a0Clauses on Loops<\/li>- 5.\u00a0
pass<\/code>\u00a0Statements<\/li>- 6. Defining Functions<\/li>
- 7. More on Defining Functions<\/li>
- 7.1. Default Argument Values<\/li>
- 7.2. Keyword Arguments<\/li>
- 7.3. Arbitrary Argument Lists<\/li>
- 7.4. Unpacking Argument Lists<\/li>
- 7.5. Lambda Expressions<\/li>
- 7.6. Documentation Strings<\/li>
- 7.7. Function Annotations <\/li><\/ul>\n\n\n\n
1.\u00a0if<\/code>\u00a0Statements<\/h2>\n\n\n\nPerhaps the most well-known statement type is the if<\/code><\/a> statement. For example:>>><\/p>\n\n\n\n>>> <\/strong>x = int(input(\"Please enter an integer: \"))\nPlease enter an integer: 42\n>>> <\/strong>if<\/strong> x < 0:\n... <\/strong> x = 0\n... <\/strong> print('Negative changed to zero')\n... <\/strong>elif<\/strong> x == 0:\n... <\/strong> print('Zero')\n... <\/strong>elif<\/strong> x == 1:\n... <\/strong> print('Single')\n... <\/strong>else<\/strong>:\n... <\/strong> print('More')\n...<\/strong>\nMore\n<\/pre>\n\n\n\nThere can be zero or more\u00a0elif<\/code><\/a>\u00a0parts, and the\u00a0else<\/code><\/a>\u00a0part is optional. The keyword \u2018elif<\/code>\u2019 is short for \u2018else if\u2019, and is useful to avoid excessive indentation. An\u00a0if<\/code>\u00a0\u2026\u00a0elif<\/code>\u00a0\u2026\u00a0elif<\/code>\u00a0\u2026 sequence is a substitute for the\u00a0switch<\/code>\u00a0or\u00a0case<\/code>\u00a0statements found in other languages.<\/p>\n\n\n\n2.\u00a0for<\/code>\u00a0Statements<\/h2>\n\n\n\nThe for<\/code><\/a> statement in Python differs a bit from what you may be used to in C or Pascal. Rather than always iterating over an arithmetic progression of numbers (like in Pascal), or giving the user the ability to define both the iteration step and halting condition (as C), Python\u2019s for<\/code> statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence. For example (no pun intended):>>><\/p>\n\n\n\n>>> <\/strong># Measure some strings:<\/em>\n... <\/strong>words = ['cat', 'window', 'defenestrate']\n>>> <\/strong>for<\/strong> w in<\/strong> words:\n... <\/strong> print(w, len(w))\n...<\/strong>\ncat 3\nwindow 6\ndefenestrate 12\n<\/pre>\n\n\n\nIf you need to modify the sequence you are iterating over while inside the loop (for example to duplicate selected items), it is recommended that you first make a copy. Iterating over a sequence does not implicitly make a copy. The slice notation makes this especially convenient:>>><\/p>\n\n\n\n
>>> <\/strong>for<\/strong> w in<\/strong> words[:]: # Loop over a slice copy of the entire list.<\/em>\n... <\/strong> if<\/strong> len(w) > 6:\n... <\/strong> words.insert(0, w)\n...<\/strong>\n>>> <\/strong>words\n['defenestrate', 'cat', 'window', 'defenestrate']\n<\/pre>\n\n\n\nWith\u00a0for\u00a0w\u00a0in\u00a0words:<\/code>, the example would attempt to create an infinite list, inserting\u00a0defenestrate<\/code>\u00a0over and over again.<\/p>\n\n\n\n3. The\u00a0range()<\/code>\u00a0Function <\/h2>\n\n\n\nIf you do need to iterate over a sequence of numbers, the built-in function\u00a0range()<\/code>\u00a0comes in handy. It generates arithmetic progressions:>>><\/p>\n\n\n\n>>> <\/strong>for<\/strong> i in<\/strong> range(5):\n... <\/strong> print(i)\n...<\/strong>\n0\n1\n2\n3\n4\n<\/pre>\n\n\n\nThe given end point is never part of the generated sequence; range(10)<\/code> generates 10 values, the legal indices for items of a sequence of length 10. It is possible to let the range start at another number, or to specify a different increment (even negative; sometimes this is called the \u2018step\u2019):<\/p>\n\n\n\nrange(5, 10)\n 5, 6, 7, 8, 9\n\nrange(0, 10, 3)\n 0, 3, 6, 9\n\nrange(-10, -100, -30)\n -10, -40, -70\n<\/pre>\n\n\n\nTo iterate over the indices of a sequence, you can combine\u00a0range()<\/code>\u00a0and\u00a0len(<\/code>)<\/code><\/a>\u00a0as follows:>>><\/p>\n\n\n\n>>> <\/strong>a = ['Mary', 'had', 'a', 'little', 'lamb']\n>>> <\/strong>for<\/strong> i in<\/strong> range(len(a)):\n... <\/strong> print(i, a[i])\n...<\/strong>\n0 Mary\n1 had\n2 a\n3 little\n4 lamb\n<\/pre>\n\n\n\nIn most such cases, however, it is convenient to use the\u00a0enumerate()<\/code>\u00a0function, see\u00a0Looping Techniques.<\/p>\n\n\n\nA strange thing happens if you just print a range:>>><\/p>\n\n\n\n
>>> <\/strong>print(range(10))\nrange(0, 10)\n<\/pre>\n\n\n\nIn many ways the object returned by\u00a0range<\/code>()<\/code><\/a>\u00a0behaves as if it is a list, but in fact it isn\u2019t. It is an object which returns the successive items of the desired sequence when you iterate over it, but it doesn\u2019t really make the list, thus saving space.<\/p>\n\n\n\nWe say such an object is\u00a0iterable<\/em>, that is, suitable as a target for functions and constructs that expect something from which they can obtain successive items until the supply is exhausted. We have seen that the\u00a0for<\/code><\/a>\u00a0statement is such an\u00a0iterator<\/em>. The function\u00a0list<\/code>()<\/code><\/a>\u00a0is another; it creates lists from iterables:>>><\/p>\n\n\n\n>>> <\/strong>list(range(5))\n[0, 1, 2, 3, 4]\n<\/pre>\n\n\n\nLater we will see more functions that return iterables and take iterables as argument.<\/p>\n\n\n\n
4.\u00a0break<\/code>\u00a0and\u00a0continue<\/code>\u00a0Statements, and\u00a0else<\/code>\u00a0Clauses on Loops<\/h2>\n\n\n\nThe break<\/code><\/a> statement, like in C, breaks out of the innermost enclosing for<\/code><\/a> or while<\/code><\/a> loop.<\/p>\n\n\n\nLoop statements may have an else<\/code> clause; it is executed when the loop terminates through exhaustion of the list (with for<\/code><\/a>) or when the condition becomes false (with while<\/code><\/a>), but not when the loop is terminated by a break<\/code><\/a> statement. This is exemplified by the following loop, which searches for prime numbers:>>><\/p>\n\n\n\n>>> <\/strong>for<\/strong> n in<\/strong> range(2, 10):\n... <\/strong> for<\/strong> x in<\/strong> range(2, n):\n... <\/strong> if<\/strong> n % x == 0:\n... <\/strong> print(n, 'equals', x, '*', n\/\/x)\n... <\/strong> break<\/strong>\n... <\/strong> else<\/strong>:\n... <\/strong> # loop fell through without finding a factor<\/em>\n... <\/strong> print(n, 'is a prime number')\n...<\/strong>\n2 is a prime number\n3 is a prime number\n4 equals 2 * 2\n5 is a prime number\n6 equals 2 * 3\n7 is a prime number\n8 equals 2 * 4\n9 equals 3 * 3\n<\/pre>\n\n\n\n(Yes, this is the correct code. Look closely: the else<\/code> clause belongs to the for<\/code><\/a> loop, not<\/strong> the if<\/code><\/a> statement.)<\/p>\n\n\n\nWhen used with a loop, the else<\/code> clause has more in common with the else<\/code> clause of a try<\/code><\/a> statement than it does that of if<\/code><\/a> statements: a try<\/code> statement\u2019s else<\/code> clause runs when no exception occurs, and a loop\u2019s else<\/code> clause runs when no break<\/code> occurs. For more on the try<\/code> statement and exceptions, see Handling Exceptions<\/a>.<\/p>\n\n\n\nThe continue<\/code><\/a> statement, also borrowed from C, continues with the next iteration of the loop:>>><\/p>\n\n\n\n>>> <\/strong>for<\/strong> num in<\/strong> range(2, 10):\n... <\/strong> if<\/strong> num % 2 == 0:\n... <\/strong> print(\"Found an even number\", num)\n... <\/strong> continue<\/strong>\n... <\/strong> print(\"Found a number\", num)\nFound an even number 2\nFound a number 3\nFound an even number 4\nFound a number 5\nFound an even number 6\nFound a number 7\nFound an even number 8\nFound a number 9<\/pre>\n\n\n\n5.\u00a0pass<\/code>\u00a0Statements<\/h2>\n\n\n\nThe pass<\/code><\/a> statement does nothing. It can be used when a statement is required syntactically but the program requires no action. For example:>>><\/p>\n\n\n\n>>> <\/strong>while<\/strong> True<\/strong>:\n... <\/strong> pass<\/strong> # Busy-wait for keyboard interrupt (Ctrl+C)<\/em>\n...<\/strong>\n<\/pre>\n\n\n\nThis is commonly used for creating minimal classes:>>><\/p>\n\n\n\n
>>> <\/strong>class<\/strong> MyEmptyClass<\/strong>:\n... <\/strong> pass<\/strong>\n...<\/strong>\n<\/pre>\n\n\n\nAnother place pass<\/code><\/a> can be used is as a place-holder for a function or conditional body when you are working on new code, allowing you to keep thinking at a more abstract level. The pass<\/code> is silently ignored:>>><\/p>\n\n\n\n>>> <\/strong>def<\/strong> initlog(*args):\n... <\/strong> pass<\/strong> # Remember to implement this!<\/em>\n...<\/strong>\n<\/pre>\n\n\n\n6. Defining Functions<\/h2>\n\n\n\n
We can create a function that writes the Fibonacci series to an arbitrary boundary:>>><\/p>\n\n\n\n
>>> <\/strong>def<\/strong> fib(n): # write Fibonacci series up to n<\/em>\n... <\/strong> \"\"\"Print a Fibonacci series up to n.\"\"\"<\/em>\n... <\/strong> a, b = 0, 1\n... <\/strong> while<\/strong> a < n:\n... <\/strong> print(a, end=' ')\n... <\/strong> a, b = b, a+b\n... <\/strong> print()\n...<\/strong>\n>>> <\/strong># Now call the function we just defined:<\/em>\n... <\/strong>fib(2000)\n0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597\n<\/pre>\n\n\n\nThe keyword def<\/code><\/a> introduces a function definition<\/em>. It must be followed by the function name and the parenthesized list of formal parameters. The statements that form the body of the function start at the next line, and must be indented.<\/p>\n\n\n\nThe first statement of the function body can optionally be a string literal; this string literal is the function\u2019s documentation string, or docstring<\/em>. (More about docstrings can be found in the section Documentation Strings<\/a>.) There are tools which use docstrings to automatically produce online or printed documentation, or to let the user interactively browse through code; it\u2019s good practice to include docstrings in code that you write, so make a habit of it.<\/p>\n\n\n\nThe execution<\/em> of a function introduces a new symbol table used for the local variables of the function. More precisely, all variable assignments in a function store the value in the local symbol table; whereas variable references first look in the local symbol table, then in the local symbol tables of enclosing functions, then in the global symbol table, and finally in the table of built-in names. Thus, global variables and variables of enclosing functions cannot be directly assigned a value within a function (unless, for global variables, named in a global<\/code><\/a> statement, or, for variables of enclosing functions, named in a nonlocal<\/code><\/a> statement), although they may be referenced.<\/p>\n\n\n\nThe actual parameters (arguments) to a function call are introduced in the local symbol table of the called function when it is called; thus, arguments are passed using call by value<\/em> (where the value<\/em> is always an object reference<\/em>, not the value of the object). 1<\/a> When a function calls another function, a new local symbol table is created for that call.<\/p>\n\n\n\nA function definition introduces the function name in the current symbol table. The value of the function name has a type that is recognized by the interpreter as a user-defined function. This value can be assigned to another name which can then also be used as a function. This serves as a general renaming mechanism:>>><\/p>\n\n\n\n
>>> <\/strong>fib\n<function fib at 10042ed0>\n