Sets and Dictionaries. Modules and File I/O

Transcription

1 Sets and Dictionaries Modules and File I/O get excited! CS GMU

2 Sets

3 some allowed set values: numbers, strings, and tuples some disallowed set values: lists, dictionaries, other sets not allowed in sets Sets set : a group of values, without duplicates. Can add/remove items, combine with common set operations (union, intersection, etc). frozenset : version of set that is immutable; allowed as values in sets (and also frozensets). The ordering of sets' values is arbitrary. Values in sets must be hashable ("immutable all the way down.")

4 Set Operations Python provides many set operations that should be familiar from mathematics. We go to the documentation here to view them directly. a course goal is being comfortable finding, reading, and using documentation J

5 POLL 7A Sets

6 Practice Problems Given sets of ints named xs, ys, and zs, find : the maximum number that is in both xs and ys. the minimum number that is in xs but not in either of ys or zs. how many unique numbers show up anywhere in the three lists. the set of numbers showing up in exactly two of the three lists. Consider these sample values. xs = { 1, 2, 3, 5, 7 } ys = { 2, 4, 5, 6 } zs = { 1, 4, 6, 7 }

7 when should we use sets? Keep sets in mind when designing your algorithms if the properties of sets (no duplicates) are useful, and the arbitrary ordering doesn't hurt, go ahead and use them directly! Occasionally, you might want to just convert to a set and back to a list to get the "no duplicates" property. Watch out for ordering, though! >>> xs = ['a','b','c','d','a','b','c'] # messy, with duplicates >>> list(set(xs)) # no duplicates (but unsorted) ['d', 'b', 'c', 'a']

8 Dictionaries (LIB 4.10)

9 dictionary example scores = {"Andrew":95, "Jerzy":82, "Mark":82 } scores dictionary: collection of key-value pairs no preserved ordering of keys (Python3.6+ is preserving insertion ordering!) keys must be unique, keys must be hashable can add/update/remove key-value pairs great way to 'index' things by non-consecutive ints key value "Andrew" 95 "Jerzy" 82 "Mark" 82

10 Dictionary Examples Syntax: {key1:val1, key2:val2, keyn:valn} empty = { } number_names = {1:"one", 2:"two", 3:"three"} name_parts = {"first":"george", "last":"mason"} random = {1:"a", (1,2,3):"abc", None:"shall pass"} Other Creation Strategies: Using dict function with keyword args, unquoted-strings as keys: dict ( a=1, b=2, c=55 ) Using dict function and sequence of length-two-sequences: stuff = [["a",1], ["b",2], ["c",55]] dict (stuff)

11 Dictionary Operations func,on/method/opera,on usage len: # of key-value pairs. len( d ) indexing: by key d[ k ] get: (use opoonal parameter default if not found) d.get(k) d.get(k, default) del: remove a key-value pair del d[ k ] in, not in: test key's presence k in d k not in d clear: remove all key-value pairs copy: create a shallow copy keys, values, items: get the keys, values, or key-val pairs pop: pop value at k (or return default) popitem(): pop any value update: insert all of another dict's key-value pairs d.clear() d.copy() d.keys() d.items() d.values() d.pop(k) d.pop(k,default) d.popitem() d_receiver.update(d_supplier)

12 Getting Keys, Values, or Key-Value Pairs >>> d = {"a":1,"b":2,"c":3} >>> d.keys() dict_keys(['c', 'b', 'a']) >>> d.values() dict_values([3, 2,1]) >>> d.items() dict_items([('c', 3), ('b', 2), ('a', 1)]) 12

13 POLL 7B Dictionaries.

14 Dictionary Iteration get one key each iteraoon (no guaranteed key order) >>> d = { "a":1, "b":2, "c":3 } >>> for k in d:... print (k, d[k])... c 3 b 2 a 1 >>> 14

15 Dictionary Iteration d = {"a":1, "b":2, "c":3} print ("by keys:") for k in d.keys(): print (k, d[k]) print ("\nby values:") for v in d.values(): print (v) print ("\nby items:") for (k,v) in d.items(): print (k,v) 15 by keys: c 3 b 2 a 1 by values: by items: c 3 b 2 a 1

16 Removing Dictionary Elements >>> d = {"a":1, "b":2, "c":3} >>> del d["a"] >>> d {'c': 3, 'b': 2} >>> d.pop("b") 2 >>> d {'c': 3} >>> d.pop("b") Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: 'b' >>> >>> d.pop("b","oops") 'oops' >>> del d["garbage"] Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: 'garbage' >>> d.popitem() # any item ('c', 3) del: >>> delete by key pop: remove by given key, receive its paired value (or default if not found) popitem: pop arbitrary key-value pair 16

17 POLL 7C Dictionaries and looping

18 Modules (LIB )

19 Modules A module is a component providing Python definitions of functions, variables or classes. basically, each.py file is a module. Think of a module as a named thing with lots of named things inside of it. 19

20 various import styles Consider the math module. It provides definioons for pi, sqrt, and cos. We use it as an example for various styles of imporong definioons. style of import statement what is accessible? sample usage import math from math import * from math import pi qualified access to all things defined in math module, as math.thing allows direct access to all things in math module. allows access to thing directly, but nothing else brought in scope squarea = math.sqrt(math.pi*(r**2)) # can't use sqrt/pi directly squarea = sqrt(pi*(r**2)) # can't use math.sqrt or math.pi area = pi * (r**2) # can't use sqrt or math.sqrt # can't use math.pi either! from math import (pi, cos, sqrt) allows direct access to these things, but not others from the math module. five = sqrt(25) circ = 2 * pi * r # can't use tan # can't use math.pi

21 POLL 7D Modules.

22 Basic File Input/Output

23 basics of file I/O programs can read and write files files might contain text or other binary data we'll only consider text files text files are essentially a python string stored on disk in file writing mode, nothing is actually written until we close the file much like how you have to save your code file to actually change what's on disk

24 two examples file_ref = open ("sample.txt") # get access to file str_contents = file_ref. read () # read entire file as string file_ref. close () # close file when done <use entire file's contents as string> fileref = open("logfile.txt", 'w' ) # let's make a file fileref. write ( "hello, file\n" ) # start wriong contents fileref. write ( "line 2\nline" ) fileref. writelines ( ["3\n", "line 4\n", "last line\n" ] ) fileref. close( ) # commits changes to file

25 phases of file interaction create a file reference by calling open(filename, mode='r') mode='r' (default): file must exist; only inspect what's there. mode='w': builds up new contents for file, saved at close() will replace any previously-existing file when closed mode='a': same as writing but adds to existing contents rather than discarding. creates new file if needed. call any useful functions allowed by the mode mode='r': read, readline, readlines mode='w': write, writelines close the file when done by calling close()

26 universal newline support various systems chose different newline representations. examples: '\n' '\r' '\r\n' python converts all variants to '\n' by default when writing files, you explicitly write any newlines you want so you can match any style you want

27 Reading Files

28 two examples file_ref = open ("sample.txt") # get access to file str_contents = file_ref. read () # read entire file as string file_ref. close () # close file when done <use entire file's contents as string> fileref = open ("myfile.txt") #access it lines = fileref. readlines( ) #read it fileref. close( ) for line in lines: #always close the file! print(line.upper(), end="") # shout it out! print ("done!") #use it! just a list of strings now

29 reading files compare to physically reading a book also, think of file's contents as a python string, indexed 0 and up you have a 'bookmark' tracking where you are, updating as you go you can read a few characters at a time, a line at a time, or the whole thing at once sample call meaning (star,ng at your 'bookmark' always) return value f. read (n) read up to n characters string f. read ( ) read all remaining characters string f. readline ( n ) read up to n characters on current line string f. readline ( ) read the rest of the current line string f. readlines ( ) read all remaining lines list of strings

30 read() test1.txt ABCDEFGHIJ qrstuvwxyz test1.txt ends in 'z\n' test2.txt ABCDEFGHIJ qrstuvwxyz test2.txt ends in 'z\n\n' >>> file = open("test1.txt") >>> file.read() ' \nABCDEFGHIJ\nqrstuvwxyz\n' >>> file.close() >>> file = open("test2.txt") >>> file.read() ' \nABCDEFGHIJ\nqrstuvwxyz\n\n' >>> file.close() 30 *** Don t forget to close file *** ( with file.close() )

31 read(x) test.txt ABCDEFGHIJ qrstuvwxyz >>> file = open("test.txt") >>> file.read(5) '01234' >>> file.read(5) # file remembered bookmark '56789' >>> file.read(4) # newline counts! '\nabc' >>> file.close() # always close the file >>> file position remembered between read() calls *** Don t forget to close file *** 31

32 readline() test.txt ABCDEFGHIJ qrstuvwxyz >>> file = open("test.txt") >>> file. readline() ' \n' >>> file. readline() 'ABCDEFGHIJ\n' >>> file. readline() 'qrstuvwxyz\n' >>> file. readline() '' >>> file. close() *** Don t forget to close file *** 32

33 readline(x) test.txt ABCDEFGHIJ qrstuvwxyz >>> file = open("test.txt") >>> file. readline(7) ' ' >>> file. readline(7) '789\n' >>> file. readline(15) 'ABCDEFGHIJ\n' >>> file. close() *** Don t forget to close file *** 33

34 readlines() test.txt ABCDEFGHIJ qrstuvwxyz >>> file = open ("test.txt") >>> file. readlines () [' \n', 'ABCDEFGHIJ\n', 'qrstuvwxyz\n'] >>> file. close () Note: newlines are preserved in the output. Last line might not have one! *** Don t forget to close file ***

35 Iteration using for Loop test.txt ABCDEFGHIJ qrstuvwxyz >>> test_file = open ("test.txt") >>> for line in test_file:... print(line, end='') ABCDEFGHIJ qrstuvwxyz >>> test_file.close() *** Don t forget to close file *** 35

36 POLL 7E Reading Files.

37 Writing Files

38 file writing methods calling write and writelines is like successive print calls, only the output goes to a file (and no newlines or separators are ever added, only exactly what you write) nothing is actually written to the file until you close it! method behavior example call write(x) writes string x to file f.write("stuff\nhere") writelines(xs) writes strings in list xs to file parts=['a\n','\b'n\c', 'd','e','f'] f.writelines(parts) 38

39 write(x) test.txt (before close) ABCDEFGHIJ qrstuvwxyz >>> file = open("test.txt","w") >>> s = "Line 1\nLine 2\nLine 3\n" >>> file.write(s) 21 >>> file.close() test.txt (ager close) Line 1 Line 2 Line 3 absolutely nothing except what you write goes into the file no newlines, spacings, or anything. you can write parts of one line in as many write calls as you need 39

40 writelines(xs) test.txt (before close) Line 1 Line 2 Line 3 >>> file = open ("test.txt","w") >>> lines = ["012345\n","ABCDEF\n","qrstu\n"] >>> file. writelines(lines) >>> file. close() test.txt (ager close) ABCDEF qrstu writelines is nothing more than a loop of calls to write(). You could write it in your sleep by now. 40

41 POLL 7F Writing Files.

42 tell and seek Methods navigate around a file by index, and (re)read/overwrite parts. tell - returns current file pointer position (as int) seek - moves file pointer to specified position test.txt ABCDEFGHIJ qrstuvwxyz >>> file = open("test.txt") >>> file. tell() # start at beginning 0 >>> file. seek(7)# absolute positioning 7 >>> file. tell() # now we're at char 7 7 >>> file. read(6)# read 6 characters. '789\nAB' >>> file. tell() # now we're at 7+6==13 13 >>> file. close()# always close the file 42