Master Python Tricks: Function Chaining, Default Argument Pitfalls, CSV I/O, and More

Function Chaining

def add(x):
    class AddNum(int):
        def __call__(self, x):
            return AddNum(self.numerator + x)
    return AddNum(x)

print(add(2)(3)(5))  # 10
print(add(2)(3)(4)(5)(6)(7))  # 27

# JavaScript version
var add = function(x){
    var addNum = function(x){
        return add(addNum + x);
    };
    addNum.toString = function(){
        return x;
    }
    return addNum;
}
add(2)(3)(5) // 10
add(2)(3)(4)(5)(6)(7) // 27

Default Argument Pitfall

def evil(v=[]):
    v.append(1)
    print(v)

evil()  # [1]
evil()  # [1, 1]

Read/Write CSV Files

import csv

with open('data.csv', 'rb') as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

# Write to CSV
with open('data.csv', 'wb') as f:
    writer = csv.writer(f)
    writer.writerow(['name', 'address', 'age'])
    data = [
        ('xiaoming', 'china', '10'),
        ('Lily', 'USA', '12')
    ]
    writer.writerows(data)

Number Base Conversion

int('1000', 2)  # 8
int('A', 16)    # 10

JSON Formatting

echo '{"k": "v"}' | python -m json.tool

List Flattening

list_ = [[1,2,3],[4,5,6],[7,8,9]]
flattened = [k for i in list_ for k in i]
# Using numpy
import numpy as np
print(np.r_[[1,2,3],[4,5,6],[7,8,9]])
# Using itertools
import itertools
print(list(itertools.chain(*list_)))
flatten = lambda x: [y for l in x for y in flatten(l)] if isinstance(x, list) else [x]
print(flatten(list_))

List Merging

a = [1,3,5,7,9]
b = [2,3,4,5,6]
c = [5,6,7,8,9]
merged = list(set().union(a, b, c))
print(merged)  # [1,2,3,4,5,6,7,8,9]

Most Common Two Letters

from collections import Counter
c = Counter('hello world')
print(c.most_common(2))  # [('l', 3), ('o', 2)]

Eval Caution

eval("__import__('os').system('rm -rf /')", {})

Matrix Transposition

matrix = [[1,2,3],[4,5,6]]
res = list(zip(*matrix))  # [(1,4), (2,5), (3,6)]

List Comprehension

[item**2 for item in lst if item % 2]
# Equivalent using map and filter
map(lambda item: item**2, filter(lambda item: item % 2, lst))
print(list(map(str, range(1,10))))

Permutations and Combinations

import itertools
# Permutations of [1,2,3,4]
for p in itertools.permutations([1,2,3,4]):
    print(''.join(str(x) for x in p))
# Combinations of 5 elements taken 3 at a time
for c in itertools.combinations([1,2,3,4,5], 3):
    print(''.join(str(x) for x in c))
# Combinations with replacement of [1,2,3] taken 2 at a time
for c in itertools.combinations_with_replacement([1,2,3], 2):
    print(''.join(str(x) for x in c))
# Cartesian product of [1,2,3] and [4,5]
for p in itertools.product([1,2,3], [4,5]):
    print(p)

Defaultdict Usage

import collections
m = collections.defaultdict(int)
print(m['a'])  # 0
m = collections.defaultdict(str)
print(m['a'])  # ''
m = collections.defaultdict(lambda: '[default value]')
print(m['a'])  # [default value]

Dictionary Reversal

m = {'a':1, 'b':2, 'c':3, 'd':4}
reversed_dict = {v: k for k, v in m.items()}
print(reversed_dict)  # {1: 'a', 2: 'b', 3: 'c', 4: 'd'}