Chapters 7&8: Files and Lists Flashcards
“Thank you/nYou’re welcome”
When printed, returns:
Thank you
You’re welcome.
/n = 1 charactcer, new line. If simply entered, will show initial version.
fhand=open(‘blah.txt’)
Opens the given text file if it exists in the python directory (in quick access). Value fhand is assigned with is only the file handle. But some subsequent functions, but not print, will be done on entire text file.
x=fhand.read()
assigns actual text of file to x as a string, not just handle
x.rstrip()
strips white space from right of the string, but NOT left. as opposed to lstrip or just strip
lift only lines starting with ‘From:’ from file, getting rid of double spacing.
fhand=open('givenfile.txt')
for line in fhand:
line=line.rstrip()
if line.startswith('From:'):
print(line)text=banana
text.find(‘c’)
returns -1, as no c in banana.
lift only lines containing given substr, getting rid of double spacing
fhand=open('givenfile.txt')
for line in fhand:
line=line.rstrip()
if line.find('given_substr')==-1:
continue
print(line)if blah blah: continue
same effect as:
if blah blah:
continue
newtext=open(‘output.txt’, ‘w’)
Write mode (second parameter, w). This creates new file, or overwrites existing file with then entered name if it exists.
writing lines
line1=’This is the first line\n’
newtext.write(line1)
returns 24, number of characters entered.
finish writing
newtext.close()
list vs string
both sequences of values: string values are characters, list values cacn be anything
values of a list
called elements or items. comma and space between them, enclosed in square brackets, strings have speech marks numbers and variables dont (as usual). Lists within lists work as you’d think - lists, within square brackets, just become another element.
print 1st item in a list
print(list[0])
reassign 3rd item in a list to x
list[2]=’x’
print all of list of oxen individually
for ox in oxen:
print(ox)
concocenating lists 1 and 2, to make {1, 2, 3] and [4, 5, 6] into one list
list1+list 2
list1*2 gives…
[1, 2, 3, 1, 2, 3]
slice operator to extract just [1, 2] from list1:
list1[0:2], or just list1[:2}
append method: adds onto end of list, e.g. add 4 to list 1 to make [1,2,3,4]
list1.append(4)
extend method,
same, but can take a list as argument and append each element onto the list to be appended. using append method appends the whole list as a single new element
Warning: list methods are not like functions or string methods - they permanently alter, x=x.blah isnt needed, and may delete content. they’re ‘void’.
Whereas with string methods this is fine, and will set the new value to a new variable, leaving previous variable unaltered. eg line=line.rstrip; for lists it would just be eg list.split(), no list= before.
while list1=list1[:2] won’t work, return list1[:2] will if eg defining a function
sort list reverse alphabetically
list1. sort()
list1. reverse()
pop method - remove an element from list, but assign it a variable
x=list1.pop(1)
x now equals 2 (element 1, the 2nd element), list1 is now [1,3,4]
element not needed? del operator
del list1[1]. can use a slice index, eg [2:6]
know the name of element but not the index?
list1.remove(2) or (‘x’), this removed the actual element 2, not the element with [2] as its index
to do to list1:
- count elements
- find highest number element
- lowest number
- total
- average
(all in print() ) len(list1) max(list1) min(list1) sum(list1) sum(list1)/len(list1) sum is numerical list only. Others can be used "on lists of strings etc where elements are comparable" - not sure what that means, doesnt count or find one with most chars.
allow user to input list:
list=list() #tells python it'll be list type While True: inp= blah list.append(inp) #then eg average=sum(list)/len(list)
convert str to list of characters:
and to list of words:
list1=list(str)
list1=str.split()
convert str to list of words, but something other than space seperating words (space is default, leave brackets empty)
list1=str.split(delimiter)
where delimiter is the object seperating them, eg - for str ‘qw-ty-fd’
join - the inverse of split.
convert list to string with chosen delimiter e.g. ‘ ‘
’ ‘.join(list1) will make [1,3,4] into ‘1 3 4’. want no delimiter? just use ‘’, ie. empty string
using split to parse lines of a file - print all the days of week from lines starting ‘From ‘ in mbox-short.txt
fhand=open('mbox-short.txt')
for line in fhand:
line=line.rstrip()
if not line.startswith('From '):
continue
words=line.split()
print(words[2]) #the day of week is 3rd word on lineequivalent vs identical
equivalent strings are identical, the same object. a=x, b=x, a is b will return True. But lists which are the same are NOT the same object, a is b will return False. They are equivalent, NOT identical
aliasing
using one variable as a reference for another. eg, list3=list1, list1 is list3 returns True. as the variable is the value of a new variable, they are identical. Best avoided if possible though with lists - altering one of them may cause an error.
method to avoid aliasing list1
list3=list1[:]
can use and or or logic operators to combine guardian code lines. e.g. with and: if len(words)<=2: continue if words[0]!='From': continue print(words[2]) #cacn be turned into:
if len(words)>2 and words[0]=='From':
print(words[2])elem
short for element of a list. eg for elem in list1:
