Input / output with Python (Python learning memo ⑤)

Output format

point

• How to write out the value (typical)
• Enter only the expression in interactive mode
  • print()
• write () * Standard output can be referred to as sys.stdout
• How to format the output
• Use string slicing and concatenation operations
• Use str.format method
• Use template class of string
• How to convert a value to a string
• repr () function
• str () function
• str () returns in a human-readable expression
• repr () produces an interpreter-readable expression

s = 'Hello, world.'
print(str(s))
# Hello, world.
print(repr(s))
# 'Hello, world.'
print(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...

#String repr()Returns string quotes and backslashes as is
hello = 'hello, world\n'
hellos = repr(hello)
print(hellos)
# 'hello, world\n'

#Every Python object is a repr()Can be converted with
print(repr((x, y, ('spam', 'eggs'))))
# (32.5, 40000, ('spam', 'eggs'))

• Can be right-justified to the specified width using str.rjust ()
• Similar methods with ljust (), center ()

rjust()Right justification using

for x in range(1, 11):
    print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ')
    print(repr(x*x*x).rjust(4))

#output
#  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

Right justification using format

for x in range(1, 11):
    print('{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x))

#output
#  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

• zfill () zero-pads the left side of the string

print('12'.zfill(5))
# 00012

print('-3.14'.zfill(7))
# -003.14

print('3.14159265359'.zfill(5))
# 3.14159265359

• str.format () replaces {} in the string with the argument value
• Can be specified by the number of the argument to be replaced, such as {1}
• There is also a way to set keyword arguments like {arg}
• There is also a way to mix and use

Various formats

#in format{}use
print('Happy birth day {} !, You are {} years old now !'.format('Kawauso', '5'))
# Happy birth day Kawauso !, You are 5 years old now !

#Argument numbering
print('Happy birth day {0} !, You are {1} years old now !'.format('Kawauso', '5'))
# Happy birth day Kawauso !, You are 5 years old now !

#Keyword argument specification
print('Happy birth day {name} !, You are {year} years old now !'.format(name='Kawauso', year='5'))
# Happy birth day Kawauso !, You are 5 years old now !

#Mixed number and keyword arguments
print('Happy birth day {0} !, You are {year} years old now !'.format('Kawauso', year='5'))
# Happy birth day Kawauso !, You are 5 years old now !

#float type notation
print('Happy birth day {name} !, You are {year:3f} now !'.format(name='Kawauso', year=5))
# Happy birth day Kawauso !, You are 5.000000 now !

#When r is used, repr()Is applied
print('Happy birth day Kawauso !, You are {!r} now !'.format(5.12345678))
# Happy birth day Kawauso !, You are 5.12345678 now !

#Notated with 3 digits after the decimal point
import math
print('Pi is about{0:.3f}Is'.format(math.pi))
#Pi is about 3.142

# %How to specify using
print('Pi is about%5.3f' % math.pi)
#Pi is about 3.142

# :You can specify the minimum width of the number of characters in the field by passing an integer after
table = {'kawauso': 100, 'mando': 200, 'banjo': 300}
for name, num in table.items():
    print('{0:10} ==> {1:10d}'.format(name, num))

# kawauso    ==>        100
# mando      ==>        200
# banjo      ==>        300

#Pass the dict[]It is convenient to access by name because you can specify it with
print('kawauso: {0[kawauso]:d}; mando: {0[mando]:d}; ' 
    'banjo: {0[banjo]}'.format(table))
# kawauso: 100; mando: 200; banjo: 300

# **You can do the same by passing it as a keyword argument using notation
print('kawauso: {kawauso:d}; mando: {mando:d}; ' 
    'banjo: {banjo}'.format(**table))
# kawauso: 100; mando: 200; banjo: 300

Reading and writing files

open()

f = open('file', 'w')

• The first argument is a character string and the file name
• The second argument is the mode
• r: Read-only
• w: Export only
• a: Added to the end of the file
• r +: literate
• Not specified: Becomes r
• b: Open in binary mode
• Normally opened in text mode
• Platform-dependent end-of-line characters are converted to \ n in text mode
• Writing reverts from \ n to platform-dependent line endings

File object method

Operate on this text file

workfile

The first line
2nd line
3rd line

f.read()

• Read from the current position to the end of the file

f = open('workfile', 'r')
print(f.read())
#output
#The first line
#2nd line
#3rd line
f.close()

f.readline()

• Read one line and go to the next line

print(f.readline())
print(f.readline())

#output
#The first line
#
#2nd line

• You can also for loop for each line in the file

for line in f:
    print(line, end='')

#output
#The first line
#
#2nd line
#
#3rd line
    
# f.write('4th line')
# print(f.read())

f.readlines()

• Store each row in an array

print(f.readlines())

#output
# ['The first line\n', '2nd line\n', '3rd line']

f.write()

• Write a string and return the number of characters below

print(f.write('4th line'))

#output
# 3

f.tell()

• Returns the current position (number of bytes from the beginning of the file)
• In binary mode, the meaning is the same, but in text mode, the number is not clear.

f.seek (offset, starting point)

• Used when you want to change the current position
• For offset, specify the number of bytes from the starting point (minus value can also be specified)
• The starting point is 0: the beginning of the file, 1: the current position, 2: the end of the file.

f = open('workfile', 'rb+')
print(f.write(b'0123456789abscef'))
# 16
print(f.seek(5))
# 5
print(f.read(1))
# b'5'
print(f.seek(-3, 2))
# 16
print(f.read(1))
# b's'

f.close()

• Call close to free system resources when you have finished working with the file.

Use with when working with files

• The file will be closed correctly when exiting a series of operations, preventing you from forgetting to close it.
• try finally block more concise than writing

with open('workfile', 'r') as f:
    read_data = f.read()

print(f.closed)
# True

Convert object to json string and save to file

point

• Serialize, deserialize and save using json package

import json
x = [1, 'kawasuo', 'list']

print(json.dumps(x))
# [1, "kawasuo", "list"]

with open('sample.json', 'w') as f:
    # sample.Write to json
    json.dump(x, f)

with open('sample.json', 'r') as f:
    # sample.Read from json
    print(json.load(f))
    # [1, "kawasuo", "list"]