Lam Hoang

Python Cheat Sheet

Hoang Lam's photo
Hoang Lam
·

2 min read

[File manipulation] Context manager

with open("welcome.txt", "r") as file:
    # 'file' refers directly to "welcome.txt"
   data = file.read()

# It closes the file automatically at the end of scope, no need for `file.close()`.

[File manipulation] Writing (append)

file = open("Hello.txt", "a") # open in append mode
file.write("Hello World again")  
file.close()

[File manipulation] Writing (overwrite)

file = open("hello.txt", "w") # open in write mode 'w'
file.write("Hello World") 

text_lines = ["First line", "Second line", "Last line"] 
file.writelines(text_lines)

file.close()

[File manipulation] Reading

file = open("hello.txt", "r") # open in read mode 'r'
file.close()

print(file.read())  # read the entire file and set the cursor at the end of file
print file.readline() # Reading one line
file.seek(0, 0) # place the cursor at the beggining of the file

Regex

import re

re.match(r'^[aeiou]', str)
re.sub(r'^[aeiou]', '?', str)
re.sub(r'(xyz)', r'\1', str)

expr = re.compile(r'^...$')
expr.match(...)
expr.sub(...)

Comprehensions

[fn(i) for i in list]            # .map
map(fn, list)                    # .map, returns iterator

filter(fn, list)                 # .filter, returns iterator
[fn(i) for i in list if i > 0]   # .filter.map

Casting

int(str)
float(str)
str(int)
str(float)
'string'.encode()

String

str[0:4]
len(str)

string.replace("-", " ")
",".join(list)
"hi {0}".format('j')
f"hi {name}" # same as "hi {}".format('name')
str.find(",")
str.index(",")   # same, but raises IndexError
str.count(",")
str.split(",")

str.lower()
str.upper()
str.title()

str.lstrip()
str.rstrip()
str.strip()

str.islower()

/* escape characters */
>>> 'doesn\'t'  # use \' to escape the single quote...
    "doesn't"
>>> "doesn't"  # ...or use double quotes instead
    "doesn't"
>>> '"Yes," they said.'
    '"Yes," they said.'
>>> "\"Yes,\" they said."
    '"Yes," they said.'
>>> '"Isn\'t," they said.'
    '"Isn\'t," they said.'

Iteration

for item in ["a", "b", "c"]:
for i in range(4):        # 0 to 3
for i in range(4, 8):     # 4 to 7
for i in range(1, 9, 2):  # 1, 3, 5, 7
for key, val in dict.items():
for index, item in enumerate(list):

Dict

dict.keys()
dict.values()
"key" in dict    # let's say this returns False, then...
dict["key"]      # ...this raises KeyError
dict.get("key")  # ...this returns None
dict.setdefault("key", 1)

Reference

Pythoncheatsheet