Python Code Snippets: Function Chaining, Default Argument Pitfalls, CSV I/O, List Operations, and More

Function Continuous Call

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‑Value Trap

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
import 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

List Flattening

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

List Merging

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

Most Common Two Letters

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

Cautious Use of eval eval("__import__('os').system('rm -rf /')", {}) Permutation Matrix

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

List Comprehension Examples

[item**2 for item in lst if item % 2]
map(lambda item: item ** 2, filter(lambda item: item % 2, lst))
list(map(str, [1,2,3,4,5,6,7,8,9]))

Permutations and Combinations

import itertools
for p in itertools.permutations([1,2,3,4]):
    print(''.join(str(x) for x in p))
for c in itertools.combinations([1,2,3,4,5], 3):
    print(''.join(str(x) for x in c))
for c in itertools.combinations_with_replacement([1,2,3], 2):
    print(''.join(str(x) for x in c))
for p in itertools.product([1,2,3], [4,5]):
    print(p)

Default Dictionary 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]'

Reverse Dictionary

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