Python beginner's note

Reference article (comprehensive and systematic summary)

• Introducing all (69) python built-in functions
• Classified AtCoder issues

pandas pandas.DataFrame pandas.DataFrame.sample

• Randomly sample rows from DataFrame, default is 1 row
• The number of lines can be specified by the first argument

pandas.Series pandas.Series.value_counts

• Returns the value of a unique element and the number of times it appears in pandas.Series

vc = df['state'].value_counts()
print(vc)
print(type(vc))
# NY    2
# CA    2
# TX    1
# Name: state, dtype: int64
# <class 'pandas.core.series.Series'>

str str.replace

• Replace strings

s = 'one two one two one'
print(s.replace(' ', '-'))
# one-two-one-two-one

• Repeat replace when replacing multiple strings
• Be careful in order as it will be replaced in the order of calling (from the left)

s = 'one two one two one'
print(s.replace('one', 'XXX').replace('two', 'YYY'))
# XXX YYY XXX YYY XXX

Replacement by slice

• There is no method to replace by specifying the position in str
• However, by dividing it with slice and concatenating it with an arbitrary character string, the same operation can be performed as a result.

s = 'abcdefghij'
print(s[:4] + 'XXX' + s[7:])
# abcdXXXhij

str.join

• Combine strings, argument is delimiter

test = ['ab', 'c', 'de']
result = ''.join(test)
print(result)
# abcde4

str.split

• Split the string
• The return value will be ** list **

str.ljust, str.rjust, str.center

• Left-justify, right-justify, and center-justify the string with the specified number of characters
• Newly generated strings are padded with spaces by default
• If the second argument is specified, it will be filled with that character

s = 'abc'
s_rjust = s.rjust(8)
print(s_rjust)
#      abc
print(s.center(8, '+'))
# ++abc+++

all

• Determine if all elements are true for iterable objects
• Effective for Set type, etc.

print(all([True, True, True]))
# True

any

• Determine if any element is true for an iterable object
• Effective for Set type, etc.

print(any([True, False, False]))
# True

find, rfind

• Get the position of any string

s = 'I am Sam'
print(s.find('Sam'))
# 5
print(s.find('XXX'))
# -1

• index and rindex have the same function, but output an error if there is no character string

list

• If you specify all the ranges in the slice of the list and set it to a minus step, the elements will be reversed.

a = [1, 2, 3, 4, 5, 6, 7, 8, 9]
print(a[::-1])
# [9, 8, 7, 6, 5, 4, 3, 2, 1]

"[]" Is removed when * list is output with *

• You can also specify the delimiter with sep = ""

Comprehension + conditional expression

• When using only if ...: [... for ... in ... if ...]
• When using if ... else ...: [... if ... else ... for ... in ...]

Comprehension + nest

• Evaluated from the outside

List=[1*i + 10*j + 100*k for k in range(2) for j in range(3) for i in range(4)]
print(List)
# [0, 1, 2, 3, 10, 11, 12, 13, 20, 21, 22, 23, 100, 101, 102, 103, 110, 111, 112, 113, 120, 121, 122, 123]

• flatten

init=[[1,2,3],[4,5],[6,7]]
[inner for outer in init for inner in outer]
# [1, 2, 3, 4, 5, 6, 7]

Delete element

list.pop

• The return value used as a method with list.pop (index) is the specified element

List=["a","b","c","d","e","f"]
print(List.pop(1))
# b
print(List)
# ['a', 'c', 'd', 'e', 'f']

list.remove

• Use as a method with list.remove (name)

List=["a","b","c","d","e","f"]
List.remove('d')
print(List)
# ['a', 'b', 'c', 'e', 'f']

del

• Use as syntax with del list [index]

List=["a","b","c","d","e","f"]
del List[1]
print(List)
# ['a', 'c', 'd', 'e', 'f']

list.insert

• Insert into list

List=["a","b","c"]
List.insert(1,'z')
print(List)
# ['a', 'z', 'b', 'c']

set

• If there are duplicate values, they will be ignored and only unique values will remain as elements.

l = [2, 2, 3, 1, 3, 4]
l_unique = list(set(l))
print(l_unique)
# [1, 2, 3, 4]

• Bit operations are also possible

s1 = set([1, 2, 3])
s2 = set([2, 3, 4])
print(s1 | s2)
# {1, 2, 3, 4}

• Set arithmetic
• Union: &
• Difference set: -
• Symmetric difference set: ^
• Sorting set with sorted () results in a list
• You can get the number of elements with len () as well as list.

ord

• Get ASCII code

ord("a")
# 74

chr

• From ASCII code to letters

chr(97)
# a

print

• If elif continues in the conditional expression of print, it can be written as follows

a,b,c,d = map(int, input().split())
print('TAKAHASHI' if b/a > d/c else 'AOKI' if b/a < d/c else 'DRAW')

float float.is_integer

• Determine if the float type number is an integer (0 after the decimal point)

f_i = 100.0
print(f_i.is_integer())
# True

numpy numpy.rehape

• Arrange vertically when converting to a two-dimensional array, etc.

mask[:, :, idx] = mask_label.reshape(256, 1600, order='F')

numpy.nditer

• Even if the dimension of ndarray increases, it can be expanded without double looping.

numpy.where

• Returns an index that satisfies the conditional expression of the first argument as ndarray

a = np.arange(20, 0, -2)
print(a)
# [20, 18, 16, 14, 12, 10,  8,  6,  4,  2]

print(np.where(a < 10))
# [6, 7, 8, 9]

bisect

• Binary search, index search from sorted list

A = [1, 2, 3, 3, 3, 4, 4, 6, 6, 6, 6]
print(A)
index = bisect.bisect_left(A, 3)
#2 (far left(Before)The insertion point of is returned)

eval

• Evaluate the argument (string) as an expression

print(eval('1 + 2'))
# 3

reduce

• Add or multiply list elements (convolution operation)

a = [-1, 3, -5, 7, -9]
print reduce(lambda x, y: x + y, a)
# -5

map, filter

• Objects created by map or filter cannot be len (), because they are italators

tqdm.tqdm

• Read the passed italator and display the whole process and current progress as a progress bar

from tqdm import tqdm
for _ in tqdm(range(100)):
  time.sleep(0.1)

Path.pathlib

• Utility, standard for all path related

from pathlib import Path
train_path = Path("../input/train_images/")
for img_name in train_path.iterdir():
    img = Image.open(img_name)

seaborn

• A wrapper to make Matplotlib's functionality more beautiful and easier

import seaborn as sns
sns.barplot(x=list(class_dict.keys()), y=list(class_dict.values()), ax=ax)

glob.glob

• If a pattern is given as an argument, the file names that match the pattern in the directory are returned as a list.
• Patterns are written in the same way as used in Unix shells

import glob
glob.glob('*.log')
# ['abc.log', 't_1.log', 't_2.log']
glob.glob(’t_*.log’)
# [t_1.log, t_2.log]