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7. Input and output
7.1. Fancier output formatting
(expression statements) print() write() sys.stdout
(formatted string literals)
fF{}Python (literal values)>>> year = 2016 >>> event = 'Referendum' >>> f'Results of the {year} {event}' 'Results of the 2016 Referendum'
str.format()method{}>>> yes_votes = 42_572_654 >>> total_votes = 85_705_149 >>> percentage = yes_votes / total_votes >>> '{:-9} YES votes {:2.2%}'.format(yes_votes, percentage) ' 42572654 YES votes 49.67%'
yes_votespercentage100 2 (Format Specification)(slicing) (concatenation) method
str() repr() SyntaxErrorstr() repr() list dictionary
>>> s = 'Hello, world.'
>>> str(s)
'Hello, world.'
>>> repr(s)
"'Hello, world.'"
>>> str(1/7)
'0.14285714285714285'
>>> x = 10 * 3.25
>>> y = 200 * 200
>>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'
>>> print(s)
The value of x is 32.5, and y is 40000...
>>> # repr()
>>> hello = 'hello, world\n'
>>> hellos = repr(hello)
>>> print(hellos)
'hello, world\n'
>>> # repr() Python
>>> repr((x, y, ('spam', 'eggs')))
"(32.5, 40000, ('spam', 'eggs'))"
string (regular expressions) string.Template $x
7.1.1. Formatted string literals
(format specifier) pi
>>> import math
>>> print(f'The value of pi is approximately {math.pi:.3f}.')
The value of pi is approximately 3.142.
':'
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
>>> for name, phone in table.items():
... print(f'{name:10} ==> {phone:10d}')
...
Sjoerd ==> 4127
Jack ==> 4098
Dcab ==> 7678
'!a' ascii()'!s' str()'!r' repr()
>>> animals = 'eels'
>>> print(f'My hovercraft is full of {animals}.')
My hovercraft is full of eels.
>>> print(f'My hovercraft is full of {animals!r}.')
My hovercraft is full of 'eels'.
= (evaluate)
>>> bugs = 'roaches'
>>> count = 13
>>> area = 'living room'
>>> print(f'Debugging {bugs=} {count=} {area=}')
Debugging bugs='roaches' count=13 area='living room'
= (self-documenting expressions) (Format Specification)
7.1.2. The string format() method
str.format() method
>>> print('We are the {} who say "{}!"'.format('knights', 'Ni'))
We are the knights who say "Ni!"
str.format() method str.format() method
>>> print('{0} and {1}'.format('spam', 'eggs'))
spam and eggs
>>> print('{1} and {0}'.format('spam', 'eggs'))
eggs and spam
str.format() method
>>> print('This {food} is {adjective}.'.format(
... food='spam', adjective='absolutely horrible'))
This spam is absolutely horrible.
>>> print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred',
... other='Georg'))
The story of Bill, Manfred, and Georg.
(dict) '[]' (key)
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
>>> print('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; '
... 'Dcab: {0[Dcab]:d}'.format(table))
Jack: 4098; Sjoerd: 4127; Dcab: 8637678
** table
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
>>> print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table))
Jack: 4098; Sjoerd: 4127; Dcab: 8637678
vars() dictionary
>>> table = {k: str(v) for k, v in vars().items()}
>>> message = " ".join([f'{k}: ' + '{' + k +'};' for k in table.keys()])
>>> print(message.format(**table))
__name__: __main__; __doc__: None; __package__: None; __loader__: ...
>>> for x in range(1, 11):
... print('{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x))
...
1 1 1
2 4 8
3 9 27
4 16 64
5 25 125
6 36 216
7 49 343
8 64 512
9 81 729
10 100 1000
7.1.3. Manual string formatting
>>> for x in range(1, 11):
... print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ')
... # 'end'
... print(repr(x*x*x).rjust(4))
...
1 1 1
2 4 8
3 9 27
4 16 64
5 25 125
6 36 216
7 49 343
8 64 512
9 81 729
10 100 1000
str.rjust() method method str.ljust() str.center() method x.ljust(n)[:n]
method str.zfill()
>>> '12'.zfill(5)
'00012'
>>> '-3.14'.zfill(7)
'-003.14'
>>> '3.14159265359'.zfill(5)
'3.14159265359'
7.1.4.
% modulo format % values format format % values (string interpolation)
>>> import math
>>> print('The value of pi is approximately %5.3f.' % math.pi)
The value of pi is approximately 3.142.
printf-style String Formatting
7.2. Reading and writing files
open() returns a file object, and is most commonly used with
two positional arguments: open(filename, mode)
>>> f = open('workfile', 'w')
mode 'r' 'w' 'a' 'r+' mode 'r'
Normally, files are opened in text mode, that means, you read and write
strings from and to the file, which are encoded in a specific encoding.
If encoding is not specified, the default is UTF-8 (see open()).
Appending a 'b' to the mode opens the file in binary mode.
Binary mode data is read and written as bytes objects.
You can not specify encoding when opening file in binary mode.
(text mode) Unix \nWindows \r\n \n \n JPEG EXE
>>> with open('workfile') as f:
... read_data = f.read()
>>> # We can check that the file has been automatically closed.
>>> f.closed
True
with f.close()
f.write() with f.close() f.write()
with f.close()
>>> f.close()
>>> f.read()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: I/O operation on closed file.
7.2.1. Methods of file objects
f
f.read(size)size size size size f.read() ''
>>> f.read()
'This is the entire file.\n'
>>> f.read()
''
f.readline() \n f.readline() '\n'
>>> f.readline()
'This is the first line of the file.\n'
>>> f.readline()
'Second line of the file\n'
>>> f.readline()
''
>>> for line in f:
... print(line, end='')
...
This is the first line of the file.
Second line of the file
list list(f) f.readlines()
f.write(string) string
>>> f.write('This is a test\n')
15
>>> value = ('the answer', 42)
>>> s = str(value) #
>>> f.write(s)
18
f.tell()
f.seek(offset, whence) offset whence whence 0 1 2 whence 0
>>> f = open('workfile', 'rb+')
>>> f.write(b'0123456789abcdef')
16
>>> f.seek(5) #
5
>>> f.read(1)
b'5'
>>> f.seek(-3, 2) #
13
>>> f.read(1)
b'd'
b seek(0, 2) f.tell() 0 offset offset
method isatty() truncate()
7.2.2. json
read() method int() '123' 123 list dictionary (parsing) (serializing)
Python JSON (JavaScript Object Notation) json Python serializing deserializing
JSON (interoperability)
x JSON
>>> import json
>>> x = [1, 'simple', 'list']
>>> json.dumps(x)
'[1, "simple", "list"]'
dumps() dump() text file f text file
json.dump(x, f)
x = json.load(f)
JSON files must be encoded in UTF-8, the default encoding for text files.
list dictionary JSON classjson