PYTHON Introduction to the Basics March 2021 S. Linner, M. Lischewski, M. Richerzhagen Forschungszentrum Jülich Member of the Helmholtz Association Table of Contents Introduction Data Types I Control Statements Functions Input/Output Errors and Exceptions Data Types II Object Oriented Programming Modules and Packages Advanced Techniques Tools Regular Expressions (optional) Summary and Outlook Member of the Helmholtz Association Slide 1 Table of Contents Introduction Data Types I Control Statements Functions Input/Output Errors and Exceptions Data Types II Object Oriented Programming Modules and Packages Advanced Techniques Tools Regular Expressions (optional) Summary and Outlook Member of the Helmholtz Association Slide 2 What is Python? Python: Dynamic programming language which supports several different programing paradigms: Procedural programming Object oriented programming Functional programming Standard: Python byte code is executed in the Python interpreter (similar to Java) → platform independent code Member of the Helmholtz Association Slide 3 Why Python? Extremly versatile language Website development, data analysis, server maintenance, numerical analysis, ... Syntax is clear, easy to read and learn (almost pseudo code) Common language Intuitive object oriented programming Full modularity, hierarchical packages Comprehensive standard library for many tasks Big community Simply extendable via C/C++, wrapping of C/C++ libraries Focus: Programming speed Member of the Helmholtz Association Slide 4 History Start implementation in December 1989 by Guido van Rossum (CWI) 16.10.2000: Python 2.0 Unicode support Garbage collector Development process more community oriented 3.12.2008: Python 3.0 Not 100% backwards compatible 2007 & 2010 most popular programming language (TIOBE Index) Recommendation for scientific programming (Nature News, NPG, 2015) Current version: Python 3.9.2 Python2 is out of support!1 1https://python3statement.org/ Member of the Helmholtz Association Slide 5 Zen of Python 20 software principles that influence the design of Python: 1 Beautiful is better than ugly. 2 Explicit is better than implicit. 3 Simple is better than complex. 4 Complex is better than complicated. 5 Flat is better than nested. 6 Sparse is better than dense. 7 Readability counts. 8 Special cases aren’t special enough to break the rules. 9 Although practicality beats purity. 10 Errors should never pass silently. 11 Unless explicitly silenced. 12 ... Member of the Helmholtz Association Slide 6 Is Python fast enough? For user programs: Python is fast enough! Most parts of Python are written in C For compute intensive algorithms: Fortran, C, C++ might be better Performance-critical parts can be re-implemented in C/C++ if necessary First analyse, then optimise! Member of the Helmholtz Association Slide 7 Hello World! #!/usr/bin/env python3 # This is a commentary print("Hello world!") hello_world.py $ python3 hello_world.py Hello world! $ $ chmod 755 hello_world.py $ ./ hello_world.py Hello world! $ Member of the Helmholtz Association Slide 8 Hello User #!/usr/bin/env python3 name = input("What’s your name? ") print("Hello", name) hello_user.py $ ./ hello_user.py What ’s your name? Rebecca Hello Rebecca $ Member of the Helmholtz Association Slide 9 Strong and Dynamic Typing Strong Typing: Object is of exactly one type! A string is always a string, an integer always an integer Counterexamples: PHP, JavaScript, C: char can be interpreted as short, void * can be everything Dynamic Typing: No variable declaration Variable names can be assigned to different data types in the course of a program An object’s attributes are checked only at run time Duck typing (an object is defined by its methods and attributes) When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.2 2James Whitcomb Riley Member of the Helmholtz Association Slide 10 Example: Strong and Dynamic Typing #!/usr/bin/env python3 number = 3 print(number , type(number )) print(number + 42) number = "3" print(number , type(number )) print(number + 42) types.py 3 <class ’int’> 45 3 <class ’str’> Traceback (most recent call last): File "types.py", line 7, in <module > print(number + 42) TypeError: can only concatenate str (not "int") to str Member of the Helmholtz Association Slide 11 Interactive Mode The interpreter can be started in interactive mode: $ python3 Python 3.7.2 (default , Mar 13 2019, 15:15:18) [GCC 5.4.0 20160609] on linux Type "help", "copyright", "credits" or "license" for more information. >>> print("hello world") hello world >>> a = 3 + 4 >>> print(a) 7 >>> 3 + 4 7 >>> Member of the Helmholtz Association Slide 12 IDLE Integrated DeveLopment Environment Part of the Python installation Member of the Helmholtz Association Slide 13 Documentation Online help in the interpreter: help(): general Python help help(obj): help regarding an object, e.g. a function or a module dir () : all used names dir(obj): all attributes of an object Official documentation: http://docs.python.org/ Member of the Helmholtz Association Slide 14 Documentation >>> help(dir) Help on built -in function dir: ... >>> a = 3 >>> dir() [’__builtins__ ’, ’__doc__ ’, ’__file__ ’, ’__name__ ’, ’a’] >>> help(a) Help on int object: ... Member of the Helmholtz Association Slide 15 Differences Python 2 – Python 3 (incomplete) Python 2 Python 3 shebang1 #!/usr/bin/python #!/usr/bin/python3 IDLE cmd1 idle idle3 print cmd (syntax) print print() input cmd (syntax) raw_input() input() unicode u"..." all strings integer type int/long int (infinite) ... hints in each chapter ⇒http://docs.python.org/3/whatsnew/3.0.html 1linux specific Member of the Helmholtz Association Slide 16 Enjoy Table of Contents Introduction Data Types I Control Statements Functions Input/Output Errors and Exceptions Data Types II Object Oriented Programming Modules and Packages Advanced Techniques Tools Regular Expressions (optional) Summary and Outlook Member of the Helmholtz Association Slide 17 Numerical Data Types int : integer numbers (infinite) float : corresponds to double in C complex : complex numbers ( j is the imaginary unit) a = 1 c = 1.0 c = 1e0 d = 1 + 0j Member of the Helmholtz Association Slide 18 Operators on Numbers Basic arithmetics: + , - , * , / hint: Python 2⇒ 1/2 = 0 Python 3⇒ 1/2 = 0.5 Div and modulo operator: // , % , divmod(x, y) Absolute value: abs(x) Rounding: round(x) Conversion: int(x) , float(x) , complex(re [, im=0]) Conjugate of a complex number: x.conjugate() Power: x ** y , pow(x, y) Result of a composition of different data types is of the “bigger” data type. Member of the Helmholtz Association Slide 19 Bitwise Operation on Integers Operations: AND: x & y OR: x | y exclusive OR (XOR) : xˆ y invert: ~x shift right n bits: x >> n shift left n bits: x << n Use bin(x) to get binary representation string of x . >>> print(bin(6),bin (3)) 0b110 0b11 >>> 6 & 3 2 >>> 6 | 3 7 >>> 6^ 3 5 >>> ~0 -1 >>> 1 << 3 8 >>> pow(2,3) 8 >>> 9 >> 1 4 >>> print(bin(9),bin(9>>1)) 0b1001 0b100 Member of the Helmholtz Association Slide 20 Strings Data type: str s = ’spam’ , s = "spam" Multiline strings: s = """spam""" No interpretation of escape sequences: s = r"sp\nam" Generate strings from other data types: str(1.0) >>> s = """ hello ... world """ >>> print(s) hello world >>> print("sp\nam") sp am >>> print(r"sp\nam") # or: print ("sp\\nam") sp\nam Member of the Helmholtz Association Slide 21 String Methods Count appearance of substrings: s.count(sub [, start[, end]]) Begins/ends with a substring? s.startswith(sub[, start[, end]]) , s.endswith(sub[, start[, end]]) All capital/lowercase letters: s.upper() , s.lower() Remove whitespace: s.strip([chars]) Split at substring: s.split([sub [,maxsplit]]) Find position of substring: s.index(sub[, start[, end]]) Replace a substring: s.replace(old, new[, count]) More methods: help(str) , dir(str) Member of the Helmholtz Association Slide 22 Lists Data type: list s = [1, "spam", 9.0, 42] , s = [] Append an element: s.append(x) Extend with a second list: s.extend(s2) Count appearance of an element: s.count(x) Position of an element: s.index(x[, min[, max]]) Insert element at position: s.insert(i, x) Remove and return element at position: s.pop([i]) Delete element: s.remove(x) Reverse list: s.reverse() Sort: s.sort([cmp[, key[, reverse]]]) Sum of the elements: sum(s) Member of the Helmholtz Association Slide 23 Tuple Data type: tuple s = 1, "spam", 9.0, 42 s = (1, "spam", 9.0, 42) Constant list Count appearance of an element: s.count(x) Position of an element: s.index(x[, min[, max]]) Sum of the elements: sum(s) Member of the Helmholtz Association Slide 24 Tuple Data type: tuple s = 1, "spam", 9.0, 42 s = (1, "spam", 9.0, 42) Constant list Count appearance of an element: s.count(x) Position of an element: s.index(x[, min[, max]]) Sum of the elements: sum(s) Multidimensional tuples and lists List and tuple can be nested (mixed): >>> A=([1 ,2 ,3] ,(1,2 ,3)) >>> A ([1, 2, 3], (1, 2, 3)) >>> A[0][2]=99 >>> A ([1, 2, 99], (1, 2, 3)) Member of the Helmholtz Association Slide 24 Lists, Strings and Tuples Lists are mutable Strings and tuples are immutable No assignment s[i] = ... No appending and removing of elements Functions like x.upper() return a new string! >>> s1 = "spam" >>> s2 = s1.upper () >>> s1 ’spam’ >>> s2 ’SPAM’ Member of the Helmholtz Association Slide 25 Operations on Sequences Strings, lists and tuples have much in common: They are sequences. Does/doesn’t s contain an element? x in s , x not in s Concatenate sequences: s + t Multiply sequences: n * s , s * n i-th element: s[i] , i-th to last element: s[-i] Subsequence (slice): s[i:j] , with step size k: s[i:j:k] Subsequence (slice) from beginning/to end: s[:-i] , s[i:] , s[:] Length (number of elements): len(s) Smallest/largest element: min(s) , max(s) Assignments: (a, b, c) = s → a = s[0] , b = s[1] , c = s[2] Member of the Helmholtz Association Slide 26 Indexing in Python positive index 0 1 2 3 4 5 6 7 8 9 10 element P y t h o n K u r s negative index -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 >>> kurs = "Python Kurs" >>> kurs [2:2] >>> kurs [2:3] t >>> kurs [2] t >>> kurs [-4:-1] Kur >>> kurs [-4:] Kurs >>> kurs [-6:-8:-1] no Member of the Helmholtz Association Slide 27 Boolean Values Data type bool: True , False Values that are evaluated to False : None (data type NoneType ) False 0 (in every numerical data type) Empty strings, lists and tuples: ” , [] , () Empty dictionaries: {} Empty sets set() All other objects of built-in data types are evaluated to True ! >>> bool([1, 2, 3]) True >>> bool("") False Member of the Helmholtz Association Slide 28 References Every object name is a reference to this object! An assignment to a new name creates an additional reference to this object. Hint: copy a list with s2 = s1[:] or s2 = list(s1) Operator is compares two references (identity), operator == compares the contents of two objects Assignment: different behavior depending on object type Strings, numbers (simple data types): create a new object with new value Lists, dictionaries, ...: the original object will be changed Member of the Helmholtz Association Slide 29 Reference - Example >>> x=1 >>> y=x >>> x is y True >>> y=2 >>> x is y False x 1 Member of the Helmholtz Association Slide 30 Reference - Example >>> x=1 >>> y=x >>> x is y True >>> y=2 >>> x is y False x y 1 Member of the Helmholtz Association Slide 30 Reference - Example >>> x=1 >>> y=x >>> x is y True >>> y=2 >>> x is y False x y 1 2 Member of the Helmholtz Association Slide 30 Reference - Example >>> x=1 >>> y=x >>> x is y True >>> y=2 >>> x is y False x y 1 2 >>> s1 = [1, 2, 3, 4] >>> s2 = s1 >>> s2[1] = 17 >>> s1 [1, 17, 3, 4] >>> s2 [1, 17, 3, 4] s1 1 2 3 4 s2 Member of the Helmholtz Association Slide 30 Reference - Example >>> x=1 >>> y=x >>> x is y True >>> y=2 >>> x is y False x y 1 2 >>> s1 = [1, 2, 3, 4] >>> s2 = s1 >>> s2[1] = 17 >>> s1 [1, 17, 3, 4] >>> s2 [1, 17, 3, 4] s1 1 17 3 4 s2 Member of the Helmholtz Association Slide 30 Groups 1 2 3 4 5 6 7 8 Member of the Helmholtz Association Slide 31 Enjoy Table of Contents Introduction Data Types I Control Statements Functions Input/Output Errors and Exceptions Data Types II Object Oriented Programming Modules and Packages Advanced Techniques Tools Regular Expressions (optional) Summary and Outlook Member of the Helmholtz Association Slide 32 The If Statement if a == 3: print("Aha!") Blocks are defined by indentation! ⇒Style Guide for Python Standard: Indentation with four spaces if a == 3: print("spam") elif a == 10: print("eggs") elif a == -3: print("bacon") else: print("something else") Member of the Helmholtz Association Slide 33 Relational Operators Comparison of content: == , < , > , <= , >= , != Comparison of object identity: a is b , a is not b And/or operator: a and b , a or b Chained comparison: a <= x < b , a == b == c , . . . Negation: not a if not (a==b) and (c<3): pass Hint: pass is a No Operation (NOOP) function Member of the Helmholtz Association Slide 34 For Loops for i in range (10): print(i) # 0, 1, 2, 3, ..., 9 for i in range(3, 10): print(i) # 3, 4, 5, ..., 9 for i in range(0, 10, 2): print(i) # 0, 2, 4, 6, 8 else: print("Loop completed.") End loop prematurely: break Next iteration: continue else is executed when loop didn’t end prematurely Member of the Helmholtz Association Slide 35 For Loops (continued) Iterating directly over sequences (without using an index): for item in ["spam", "eggs", "bacon"]: print(item) The range function can be used to create a list: >>> list(range(0, 10, 2)) [0, 2, 4, 6, 8] If indexes are necessary: for (i, char) in enumerate("hello world"): print(i, char) Member of the Helmholtz Association Slide 36 While Loops i = 0 while i < 10: i += 1 break and continue work for while loops, too. Substitute for do-while loop: while True: # important code if condition: break Member of the Helmholtz Association Slide 37 Enjoy Table of Contents Introduction Data Types I Control Statements Functions Input/Output Errors and Exceptions Data Types II Object Oriented Programming Modules and Packages Advanced Techniques Tools Regular Expressions (optional) Summary and Outlook Member of the Helmholtz Association Slide 38 Functions def add(a, b): """ Returns the sum of a and b.""" mysum = a + b return mysum >>> result = add(3, 5) >>> print(result) 8 >>> help(add) Help on function add in module __main__: add(a, b) Returns the sum of a and b. Member of the Helmholtz Association Slide 39 Return Values and Parameters Functions accept arbitrary objects as parameters and return values Types of parameters and return values are unspecified Functions without explicit return value return None def hello_world (): print("Hello World!") a = hello_world () print(a) my_program.py $ python3 my_program.py Hello World! None Member of the Helmholtz Association Slide 40 Multiple Return Values Multiple return values are realised using tuples or lists: def foo (): a = 17 b = 42 return (a, b) ret = foo() (x, y) = foo() Member of the Helmholtz Association Slide 41 Optional Parameters – Default Values Parameters can be defined with default values. Hint: It is not allowed to define non-default parameters after default parameters def fline(x, m=1, b=0): # f(x) = m*x + b return m*x + b for i in range (5): print(fline(i),end=" ") #force newline print() for i in range (5): print(fline(i,-1,1),end=" ") plot_lines.py $ python3 plot_lines.py 0 1 2 3 4 1 0 -1 -2 -3 Hint: end in print defines the last character, default is linebreak Member of the Helmholtz Association Slide 42 Positional Parameters Parameters can be passed to a function in a different order than specified: def printContact(name ,age ,location ): print("Person: ", name) print("Age: ", age , "years") print("Address: ", location) printContact(name="Peter Pan", location="Neverland", age =10) displayPerson.py $ python3 displayPerson.py Person: Peter Pan Age: 10 years Address: Neverland Member of the Helmholtz Association Slide 43 Functions are Objects Functions are objects and as such can be assigned and passed on: >>> a = float >>> a(22) 22.0 >>> def foo(fkt): ... print(fkt (33)) ... >>> foo(float) 33.0 >>> foo(str) 33 >>> foo(complex) (33+0j) Member of the Helmholtz Association Slide 44 Online Help: Docstrings Can be used in function, modul, class and method definitions Is defined by a string as the first statement in the definition help(...) on python object returns the docstring Two types of docstrings: one-liners and multi-liners def complex(real =0.0, imag =0.0): """ Form a complex number. Keyword arguments: real -- the real part (default 0.0) imag -- the imaginary part (default 0.0) """ ... Member of the Helmholtz Association Slide 45 Functions & Modules Functions thematically belonging together can be stored in a separate Python file. (Same for objects and classes) This file is called module and can be loaded in any Python script. Multiple modules available in the Python Standard Library (part of the Python installation) Command for loading a module: import <filename> (filename without ending .py) import math s = math.sin(math.pi) More information for standard modules and how to create your own module see chapter Modules and Packages on slide 91 Member of the Helmholtz Association Slide 46 Enjoy Table of Contents Introduction Data Types I Control Statements Functions Input/Output Errors and Exceptions Data Types II Object Oriented Programming Modules and Packages Advanced Techniques Tools Regular Expressions (optional) Summary and Outlook Member of the Helmholtz Association Slide 47 String Formatting Format string + class method x.format() “replacement fields”: curly braces around optional arg_name (default: 0,1,2,. . .) print("The answer is {0:4d}".format (42)) ’The answer is 42’ s = "{0}: {1:08.3f}".format("spam", 3.14) ’spam: 0003.140 ’ format purpose default: string m.nf floating point: m filed size, n digits after the decimal point (6) m.ne floating point (exponential): m filed size, 1 digit before and n digits behind the decimal point (default: 6) m.n% percentage: similar to format f, value ∗ 100 with finalizing ’%’ md Integer number: m field size (0m⇒leading “0”) format d can be replaced by b (binary), o (octal) or x (hexadecimal) Member of the Helmholtz Association Slide 48 Literal String Interpolation (f-strings) Provides a way to embed expressions inside string literals, using a minimal syntax Is a literal string, prefixed with ’f’, which contains expressions inside braces Expressions are evaluated at runtime and replaced with their values. >>> name = "Martin" >>> age = 50 >>> f"My name is {name} and my age next year is {age +1}" ’My name is Martin and my age next year is 51’ >>> value = 12.345 >>> f"value={value :5.2f}" ’value =12.35 ’ Hint: Since Python 3.6! Member of the Helmholtz Association Slide 49 String Formatting (deprecated, Python 2 only) String formatting similar to C: print "The answer is %4i." % 42 s = "%s: %08.3f" % ("spam", 3.14) Integer decimal: d, i Integer octal: o Integer hexadecimal: x, X Float: f, F Float in exponential form: e, E, g, G Single character: c String: s Use %% to output a single % character. Member of the Helmholtz Association Slide 50 Command Line Input User input in Python 3: user_input = input("Type something: ") User input in Python 2: user_input = raw_input("Type something: ") Hint: In Python 2 is input("...") ⇐⇒ eval(raw_input("...")) Command line parameters: import sys print(sys.argv) params.py $ python3 params.py spam [’params.py’, ’spam ’] Member of the Helmholtz Association Slide 51 Files file1 = open("spam.txt", "r") file2 = open("/tmp/eggs.json", "wb") Read mode: r Write mode (new file): w Write mode, appending to the end: a Handling binary files: e.g. rb Read and write (update): r+ for line in file1: print(line) Member of the Helmholtz Association Slide 52 Operations on Files Read: f.read([size]) Read a line: f.readline() Read multiple lines: f.readlines([sizehint]) Write: f.write(str) Write multiple lines: f.writelines(sequence) Close file: f.close() file1 = open("test.txt", "w") lines = ["spam\n", "eggs\n", "ham\n"] file1.writelines(lines) file1.close () Python automatically converts \n into the correct line ending! Member of the Helmholtz Association Slide 53 The with statement File handling (open/close) can be done by the context manager with . (⇒section Errors and Exceptions on slide 65). with open("test.txt") as f: for line in f: print(line) After finishing the with block the file object is closed, even if an exception occurred inside the block. Member of the Helmholtz Association Slide 54 Enjoy Table of Contents Introduction Data Types I Control Statements Functions Input/Output Errors and Exceptions Data Types II Object Oriented Programming Modules and Packages Advanced Techniques Tools Regular Expressions (optional) Summary and Outlook Member of the Helmholtz Association Slide 55 Syntax Errors, Indentation Errors Parsing errors: Program will not be executed. Mismatched or missing parenthesis Missing or misplaced semicolons, colons, commas Indentation errors print("I’m running ...") def add(a, b) return a + b add.py $ python3 add.py File "add.py", line 2 def add(a, b) ^ SyntaxError: invalid syntax Member of the Helmholtz Association Slide 56 Exceptions Exceptions occur at runtime: import math print("I’m running ...") math.foo() print("I’m still running ...") error.py $ python3 error.py I’m running ... Traceback (most recent call last): File "error.py", line 3, in <module > math.foo() AttributeError: module ’math ’ has no attribute ’foo ’ Member of the Helmholtz Association Slide 57 Handling Exceptions (1) try: s = input("Enter a number: ") number = float(s) except ValueError: print("That’s not a number!") except block is executed when the code in the try block throws an according exception Afterwards, the program continues normally Unhandled exceptions force the program to exit. Handling different kinds of exceptions: except (ValueError , TypeError , NameError ): Built-in exceptions: http://docs.python.org/library/exceptions.html Member of the Helmholtz Association Slide 58 Handling Exceptions (2) try: s = input("Enter a number: ") number = 1/ float(s) except ValueError: print("That’s not a number!") except ZeroDivisionError: print("You can’t divide by zero!") except: print("Oops , what’s happened?") Several except statements for different exceptions Last except can be used without specifying the kind of exception: Catches all remaining exceptions Careful: Can mask unintended programming errors! Member of the Helmholtz Association Slide 59 Handling Exceptions (3) else is executed if no exception occurred finally is executed in any case try: f = open("spam") except IOError: print("Cannot open file") else: print(f.read ()) f.close() finally: print("End of try.") Member of the Helmholtz Association Slide 60 Exception Objects Access to exception objects: EnvironmentError ( IOError , OSError ): Exception object has 3 attributes ( int , str , str ) Otherwise: Exception object is a string try: f = open("spam") except IOError as e: print(e.errno , e.filename , e.strerror) print(e) spam_open.py $ python3 spam_open.py 2 spam No such file or directory [Errno 2] No such file or directory: ’spam ’ Member of the Helmholtz Association Slide 61 Exceptions in Function Calls draw() rectangle() line() Exception! Function calls another function. That function raises an exception. Is exception handled? No: Pass exception to calling function. Member of the Helmholtz Association Slide 62 Raising Exceptions Passing exceptions on: try: f = open("spam") except IOError: print("Problem while opening file!") raise Raising exceptions: def gauss_solver(matrix ): # Important code raise ValueError("Singular matrix") Member of the Helmholtz Association Slide 63 Exceptions vs. Checking Values Beforehand Exceptions are preferable! def square(x): if type(x) == int or type(x) == float: return x ** 2 else: return None What about other numerical data types (complex numbers, own data types)? Better: Try to compute the power and catch possible exceptions! → Duck-Typing Caller of a function might forget to check return values for validity. Better: Raise an exception! Member of the Helmholtz Association Slide 64 Exceptions vs. Checking Values Beforehand Exceptions are preferable! def square(x): if type(x) == int or type(x) == float: return x ** 2 else: return None def square(x): return x ** 2 ... try: result = square(value) except TypeError: print(" ’{0}’: Invalid type".format(value)) Member of the Helmholtz Association Slide 64 The with Statement Some objects offer context management 3 , which provides a more convenient way to write try ... finally blocks: with open("test.txt") as f: for line in f: print(line) After the with block the file object is guaranteed to be closed properly, no matter what exceptions occurred within the block. 3Class method __enter__(self) will be executed at the beginning and class method __exit__(...) at the end of the context Member of the Helmholtz Association Slide 65 Enjoy Table of Contents Introduction Data Types I Control Statements Functions Input/Output Errors and Exceptions Data Types II Object Oriented Programming Modules and Packages Advanced Techniques Tools Regular Expressions (optional) Summary and Outlook Member of the Helmholtz Association Slide 66 Sets Set: unordered, no duplicated elements s = {"a", "b", "c"} alternative s = set([sequence]) , required for empty sets. Constant set: s = frozenset([sequence]) e.g. empty set: empty = frozenset() Subset: s.issubset(t) , s <= t , strict subset: s < t Superset: s.issuperset(t) , s >= t , strict superset: s > t Union: s.union(t) , s | t Intersection: s.intersection(t) , s & t Difference: s.difference(t) , s - t Symmetric Difference: s.symmetric_difference(t) , sˆ t Copy: s.copy() As with sequences, the following works: x in s , len(s) , for x in s , s.add(x) , s.remove(x) Member of the Helmholtz Association Slide 67 Dictionaries Other names: Hash, Map, Associative Array Mapping of key→ value Keys are unordered >>> store = { "spam": 1, "eggs": 17} >>> store["eggs"] 17 >>> store["bacon"] = 42 >>> store {’eggs’: 17, ’bacon’: 42, ’spam’: 1} Iterating over dictionaries: for key in store: print(key , store[key]) Compare two dictionaries: store == pool Not allowed: > , >= , < , <= Member of the Helmholtz Association Slide 68 Operations on Dictionaries Delete an entry: del(store[key]) Delete all entries: store.clear() Copy: store.copy() Does it contain a key? key in store Get an entry: store.get(key[, default]) Remove and return entry: store.pop(key[, default]) Remove and return arbitrary entry: store.popitem() Member of the Helmholtz Association Slide 69 Operations on Dictionaries Delete an entry: del(store[key]) Delete all entries: store.clear() Copy: store.copy() Does it contain a key? key in store Get an entry: store.get(key[, default]) Remove and return entry: store.pop(key[, default]) Remove and return arbitrary entry: store.popitem() Views on Dictionaries Create a view: items() , keys() and values() List of all (key, value) tuples: store.items() List of all keys: store.keys() List all values: store.values() Caution: Dynamical since Python 3 Member of the Helmholtz Association Slide 69 Views Behavior: Python 2.X versus Python 3.X Python 2 (static) >>> mdict={"a":2, "d":5} >>> mdict {’a’: 2, ’d’: 5} >>> s=mdict.items () >>> for i in s: print(i) (’a’, 2) (’d’, 5) >>> mdict[’a’]=-1 >>> mdict {’a’: -1, ’d’: 5} >>> for i in s: print(i) (’a’, 2) (’d’, 5) Python 3 (dynamic) >>> mdict={"a":2, "d":5} >>> mdict {’a’: 2, ’d’: 5} >>> s=mdict.items () >>> for i in s: print(i) (’a’, 2) (’d’, 5) >>> mdict[’a’]=-1 >>> mdict {’a’: -1, ’d’: 5} >>> for i in s: print(i) (’a’, -1) (’d’, 5) Member of the Helmholtz Association Slide 70 Enjoy Table of Contents Introduction Data Types I Control Statements Functions Input/Output Errors and Exceptions Data Types II Object Oriented Programming Modules and Packages Advanced Techniques Tools Regular Expressions (optional) Summary and Outlook Member of the Helmholtz Association Slide 71 Object Oriented Programming (OOP) So far: procedural programming Data (values, variables, parameters, . . .) Functions taking data as parameters and returning results Alternative: Group data and functions belonging together to form custom data types → Extensions of structures in C/Fortran Member of the Helmholtz Association Slide 72 Using Simple Classes as Structs class Point: pass p = Point () p.x = 2.0 p.y = 3.3 my_point.py Class: Custom date type (here: Point ) Object: Instance of a class (here: p ) Attributes (here x , y ) can be added dynamically Hint: pass is a No Operation (NOOP) function Member of the Helmholtz Association Slide 73 Classes - Constructor class Point: def __init__(self , x, y): self.x = x self.y = y p = Point (2.0, 3.0) print(p.x, p.y) p.x = 2.5 p.z = 42 my_point.py __init__ : Is called automatically after creating an object Member of the Helmholtz Association Slide 74 Methods on Objects import math class Point: def __init__(self , x, y): self.x = x self.y = y def norm(self): n = math.sqrt(self.x**2 + self.y**2) return n p = Point (2.0, 3.0) print(p.x, p.y, p.norm ()) my_point.py Method call: automatically sets the object as first parameter → traditionally called self Careful: Overloading of methods not possible! Member of the Helmholtz Association Slide 75 Converting Objects to Strings Default return value of str(...) for objects of custom classes: >>> p = Point (2.0, 3.0) >>> print(p) # --> print(str(p)) <__main__.Point instance at 0x402d7a8c > Member of the Helmholtz Association Slide 76 Converting Objects to Strings Default return value of str(...) for objects of custom classes: >>> p = Point (2.0, 3.0) >>> print(p) # --> print(str(p)) <__main__.Point instance at 0x402d7a8c > This behaviour can be overwritten: class Point: [...] def __str__(self): return "({0}, {1})".format(self.x, self.y) my_point.py >>> print(p) (2.0, 3.0) Member of the Helmholtz Association Slide 76 Comparing Objects Default: == checks for object identity of custom objects. >>> p1 = Point (2.0, 3.0) >>> p2 = Point (2.0, 3.0) >>> p1 == p2 False Member of the Helmholtz Association Slide 77 Comparing Objects Default: == checks for object identity of custom objects. >>> p1 = Point (2.0, 3.0) >>> p2 = Point (2.0, 3.0) >>> p1 == p2 False This behaviour can be overwritten: class Point: [...] def __eq__(self , other): return (self.x == other.x) and (self.y == other.y) my_point.py >>> p1 == p2 # Check for equal values True >>> p1 is p2 # Check for identity False Member of the Helmholtz Association Slide 77 Operator overloading More relational operators: < : __lt__(self, other) <= : __le__(self, other) != : __ne__(self, other) > : __gt__(self, other) >= : __ge__(self, other) Numeric operators: + : __add__(self, other) - : __sub__(self, other) * : __mul__(self, other) ... Member of the Helmholtz Association Slide 78 Emulating Existing Data Types Classes can emulate built-in data types: Numbers: arithmetics, int(myobj) , float(myobj) , . . . Functions: myobj(...) Sequences: len(myobj) , myobj[...] , x in myobj , ... Iteratores: for i in myobj See documentation: http://docs.python.org/3/reference/datamodel.html Member of the Helmholtz Association Slide 79 Class Variables Have the same value for all instances of a class: class Point: count = 0 # Count all point objects def __init__(self , x, y): Point.count += 1 #self.__class__.count += 1 [...] my_point.py >>> p1 = Point(2, 3); p2 = Point(3, 4) >>> p1.count 2 >>> p2.count 2 >>> Point.count 2 Member of the Helmholtz Association Slide 80 Class Methods and Static Methods class Spam: spam = "I don’t like spam." @classmethod def cmethod(cls): print(cls.spam) @staticmethod def smethod (): print("Blah blah.") spam.py Spam.cmethod () Spam.smethod () s = Spam() s.cmethod () s.smethod () Member of the Helmholtz Association Slide 81 Inheritance (1) There are often classes that are very similar to each other. Inheritance allows for: Hierarchical class structure (is-a-relationship) Reusing of similar code Example: Different types of phones Phone Mobile phone (is a phone with additional functionality) Smart phone (is a mobile phone with additional functionality) Member of the Helmholtz Association Slide 82 Inheritance (2) class Phone: def call(self): pass class MobilePhone(Phone): def send_text(self): pass MobilePhone now inherits methods and attributes from Phone. h = MobilePhone () h.call() # inherited from Phone h.send_text () # own method Member of the Helmholtz Association Slide 83 Overwriting Methods Methods of the parent class can be overwritten in the child class: class MobilePhone(Phone): def call(self): self.find_signal () Phone.call(self) Member of the Helmholtz Association Slide 84