Python For Me
- list - array
- dict - hash, associative array
- tuple - immutable list
- set - unique elements
pass # place holder, does nothing
exit()
Print and string concat:
x = 5
print("test " + str(x))
print("test", x)
Split string:
a = x.splitlines() # split string by lines
Empty list/dict:
if not a: # empty list <== use this
if len(a) == 0: # empty list
if len(s) == 0: # empty string
if s == "": # empty string
if s.strip() == "": # only spaces
if s is None: # undefined
if not s: # undefined or empty <== use this
- None - assigned from function taht doesn’t return, dict value that doesn’t exist, etc
Check length:
len("test" # string length
len(a) # list length
type(a) # check type
Sleep:
import time
time.sleep(5)
Control Flow
if x < 5:
print('less than 5')
x = x + 2
elif x == 50:
print('Medium')
elif x == 100:
print('Large')
else:
print('Default')
More:
if x != "taco": # negative string compare
pass
if x < 10 and x > 5: # logical and
pass
if x > 10 and x < 5: # logical or
pass
if "taco" not in food: # not in list/collection
pass
if x is y: # same object?
pass
print("yes") if "taco" in food else print("no") # ternary operators
("no","yes")["taco" in food]
(print("no"),print("yes"))["taco" in food]
Match:
match option: # like switch statement, stops after first match
case 123:
print("First Option")
case 456:
print("Second Option")
Loops
Range:
range(5, 15) # 5 6 7 8 9 10 11 12 13 14
range(5, 25, 2) # 5 7 9 11 13 15 17 19 21 23
sum(range(4))
For loop:
for i in a:
print(i)
for i in range(5):
print(i)
for i in range(len(a)):
print(i, a[i])
for i in range(0, 9, 2): # Python C style for loop
print(i)
for i in a:
if i == "pizza":
continue # skip to next iteration
if i == "taco":
break # break out of loop
else:
print("no tacos found") # if no break during loop
While loop:
x = 0
while x < 50:
print("x: " + str(x))
x = x + 1
while True:
print("x")
Tuples
t = () # empty tuple
t = 'pizza', # tuple with one value
t = ('pizza',) # tuple with one value
t = ('pizza') # NOT a tuple, needs a comma
t = 'burger', 'taco', 'pizza'
t = ('buger', 'taco', 'pizza')
x = t, ('mustard', 'hot sauce') # nested tuple
t[0] # index a tuple
a, b, c = t # unpack
List Stuff
List comprehension:
newlist = [x for x in fruits if "a" in x]
Enumerate a list with index and value:
a = [ 'a', 'b', 'c' ]
for i in a:
print(i)
for i,j in enumerate(a): # index and value
print(i " " j)
Initialize dict or list:
a = []
d = {}
Populate list with regex split:
a = re.split(",", "a,b,c,d,e,f,g") # populate
List slicing:
a[-1] # last element
a[2:7] # 2-7 exclude last
a[2:7:2 ] # 2-3, step 2, exclude last
a[7:2:-1 ] # count backwards
a[:5] # begin to 5
a[5:] # 5 to end
a[:] # entire list
a[::-1] # reverse list
Sorting:
sorted(a) # sort a list, return a new list, works on all iterables
sorted("test") # sort a string ( NOT with sort funstion though )
a.sort() # sort in place
a.sort(reverse=True)
a.sort(key = str.lower)
a.reverse() # reverse list in place
reversed(a) # works on any iterator, returns an iterator
list(reversed(a)) # convert to list
b = set(a) # create set from list ( unique items only )
Sort with custom function:
def test1(x):
return len(x)
a.sort(key=test1)
Check if exists in list:
if 5 in a:
List operations:
a[3] = "x" # assign value
a.insert(1, "taco") # insert at index ( between 0 and 1
a[1:3] = ["a", "b"] # replace slice
a.append("x") # append element
a.extend(a2) # append list or any iterable
a = a a2 # concatenate lists
a.append(a2) # append entire list as a sublist
a.remove("x") # remove first match
a.pop() # remove/return last
a.pop(5) # remove/return at index
del a[3] # delete
del a # delete entire list
a.clear() # delete all elements ( clear list )
del a[:] # delete all elements ( clear list )
a.index("x") # get index of first match
a.count("x") # number of matches
a2 = a.copy() # copy list, return shallow copy in place, same as: a[:]
Zip:
meal = ["breakfast", "lunch", "dinner"]
food = ["pizza", "noodle", "burger"]
for a, b in zip(meal, food):
print(a + ": " + b )
List Comprehensions
a = [0, 1, 2, 3, 4, 5]
b = [9, 8, 7, 6, 5, 4]
[x*2 for x in a] # 1 for loop ( basic )
[x+3 for x in [1,2,3]] # 1 for loop ( basic )
[x for x in a if x > 1] # for and if
[abs(x) for x in a] # abs example
[x + y for x in a for y in b] # 2 for loops - left loop, then right
[(x,x*5) for x in a] # return list of tuples
[[x,x*5] for x in a] # return list of lists
[(x, y) for x in a for y in b if x != y] # 2 for loop, if, return tuple - left loop, then right
b = [[j * i for j in range(len(a))]for i in a] # nested - right loop then left
Queues with deque
from collections import deque
my_q = deque(["pizza", "burger", "sushi"])
my_q.append("taco")
my_q.append("noodle")
my_q.popleft()
my_q.popleft()
Dict / Hash Stuff
Enumerate Dict / Hash:
d = {'a': 123, 'b': 456, 'c':789, 'd': 000}
list(d) # list of keys in insertion order
sorted(d) # keys in sorted order
for i, k in enumerate(d): # index / key
print(i, k)
for k, v in d.items(): # key / value
print(i, k, v)
for i, (k, v) in enumerate(d.items()): # index / key / value
print(i, k, v)
{x: x*5+3 for x in (22, 45, 67, 1)} # Dict comprehension:
File Management
import os # OS module
import shutil # Some utils
shutil.copy(src, dst) # Copy file
os.rename(src, dst) # rename file
os.unlink("/tmp/foo.txt") # delete file
os.listdir("/dir") # List files
os.mkdir(path[, mode]) # Create directory
shutil.copy(src, dst) # copy content and permission
os.rmdir(path) # remove dir, only empty dirs
os.removedirs(path) # remove empty dir, recursive
Get home dir:
import os
print(os.path.expanduser('~'))
from pathlib import Path
print(Path.home())
Globbing:
import glob
a = glob.glob('*.txt')
for i in a:
print(i)
File IO
| ‘r’ | read - default |
| ‘w’ | write |
| ‘rb’ | read binary |
| ‘wb’ | write binary |
| ‘a’ | append |
| ‘a ‘ | append but also read and seek backwards but always write to end |
f = open('workfile', 'w') # r or w
f.write('This is a test\n') # write
f.read() # entire file
f.readline() # line at a time
f.readlines() # entire file, as list
f.close() # close it
with open("data1.txt") as f1:
data1 = f1.readlines()
with open("data1.txt") as f1:
data1 = f1.read()
data1 = [ x.strip() for x in data1 ] # strip newline and padding
Regular Expressions
Regular Expressions Characters:
\d decimal digit [0-9]
\s whitespace [ \t\n\r\f\v]
\w alphanumeric [a-zA-Z0-9_]
\D non decimal
\S non whitespace
\W non-alphanumeric
probably more .....
^ start
$ end
. any character except newline
* 0 or more
1 or more
? 0 or 1
{0,1}
Flags
re.S Make . match any char, including newlines
re.I Do case-insensitive matches
re.M Multi-line matching, affecting ^ and $
import re
m = re.match(r'cups', data1) # first match, check BEGINNING, ret: match obj or null
m = re.search(r'cups', data1) # first match, check all lines, ret: match obj or null
m = re.search(r'((sys)t)(emd)', data1) # using separate and nested groups
m.group() # match found - first match
m.groups() # groups if using grouping - from first match
m = re.findall(r'systemd', data1) # find all matches, return list of matched strings
m = re.findall(r'((sys)temd)', data1) # find all matches, return groups as list of tuples, NOT matched strings
# need more than one group
print(m)
a = re.split(r'\s', data1) # split into list ( ex: by space )
a = re.split(r'\W ', data1) # split out list of words
x = re.sub( r'xxxx', 'abc', 'xxxx xxxx')
x = re.sub( r'(blue|white|red)', 'colour', 'blue socks and red shoes')
x = re.sub( r'xxxx', '5', 'process_data.sh input_xxxx.dat output_xxxx.dat')
# check if match found:
if m:
print 'Match found: ', m.group()
# compile first for efficiency ( if you plan to loop / iterate a lot ):
p = re.compile(r”match this”)
m = p.search('string goes here')
a = p.split('test test')
x = p.sub( 'colour', 'blue socks and red shoes')
p = re.compile('ab*', re.I | re.M) # case insensitive, multi-line
Function:
- Positional args need to come before keyword args
- Otherwise keyword args can be out of order
- Args without a default can still be used as a keyword arg
Arbitrary argument lists:
| **name | will hold a dictionary with all keyword args that aren’t formal args |
| *name | tuple containing all positional args that aren’t formal args |
def function1(one, two='Yes', *name2, **name3):
value = str(one) + str(two)
return value
function1("a", "b")
function1("a", "b", 1, 2, 3, x="4", y="3")
function1("a", "b", 1, 2, 3, 4, 3)
z1 = ["a", "b"]
z2 = {'one':"a", 'two':"b"}
function1(*z1) # pass list to be unpacked
function1(**z2) # pass dict to be unpacked
Function Annotations - types for each argument and type for return value:
def test3(a: str, b: str = 'xyz') -> str:
print("Annotations: ", test3.__annotations__)
Lambda
- anonymous function
- assigned to a variable that can be passed around
- single expression
- takes args
# lambda args : expression
x = lambda a : a * 5
print( x(3) )
x = lambda a : "Value is: " + str(a * 2)
print(x(5))
Lambda to check if element in dict and return default value:
t = { "a": "test1", "b": "test2" }
b = lambda a,i : a[i] if i in a else "default"
b(t,"c")
Function to generate a lambda:
def test1(n):
return lambda x: x + n
x = test1(42)
x(0)
Subprocess / OS Commands:
https://www.datacamp.com/tutorial/python-subprocess <==== review more
import subprocess
subprocess.check_output( "uname -a", stderr=subprocess.STDOUT, shell=True )
subprocess.run(["ls", "-l"])
cmd1 = "ls -l /tmp | awk '{print $2,$4}'"
x = subprocess.run(cmd1, shell=True)
x = subprocess.run(cmd1, shell=True, capture_output=True, text=True)
check # raise error on non-zero timeout # specify timeout
x.returncode
x.args
x.stdout
x.stderr
CSV:
https://realpython.com/python-csv/
XLS
JSON
import json
x = '{ "name":"John", "age":30, "city":"New York"}'
y = json.loads(x) # string to object
print(y["age"])
x = {
"name": "John",
"age": 30,
"city": "New York"
}
y = json.dumps(x) # object to string
print(y)
Error Handling - Try / Except
if x < 0:
raise Exception("Sorry, no numbers below zero")
if not type(x) is int:
raise TypeError("Only integers are allowed")
try:
print(x)
except NameError:
print("Variable x is not defined")
except:
print("Something else went wrong")
try:
print("Hello")
except:
print("Something went wrong")
else:
print("Nothing went wrong")
try:
print(x)
except:
print("Something went wrong")
finally:
print("The 'try except' is finished")
nesting:
try:
f = open("demofile.txt")
try:
f.write("Lorum Ipsum")
except:
print("Something went wrong when writing to the file")
finally:
f.close()
except:
print("Something went wrong when opening the file")
input
# python 3.6
username = input("Enter username:")
print("Username is: " username)
# python 2.7
username = raw_input("Enter username:")
print("Username is: " username)
Args
import sys
print( sys.argv[0] ) # current script
if len(sys.argv) >= 1:
print( sys.argv[1] ) # first arg
Classes and Objects
if hasattr(instance, 'tags'): # Check if object has an attribute
....
print(type(instance.tags)) # Show type
print(instance.__dict__ # Show members of a class