Skip to content

Instantly share code, notes, and snippets.

@tonyyang-svail

tonyyang-svail/python+notes.md

Last active August 3, 2018 00:36
Show Gist options
  • Save tonyyang-svail/fb1a4747243596d7f2256a3d887603cc to your computer and use it in GitHub Desktop.
Save tonyyang-svail/fb1a4747243596d7f2256a3d887603cc to your computer and use it in GitHub Desktop.
Download ZIP
Raw
python+notes.md

Python Notes

Data Structures

List

from __future__ import print_function
import os
import psutil

process = psutil.Process(os.getpid())
print('init:', process.memory_info().rss / 1024 / 1024, 'MB')    # 11 MB

x = range(2**25)
print('x', process.memory_info().rss / 1024 / 1024, 'MB')        # 1054 MB

y = x[1:] # deep copy
print('x, y', process.memory_info().rss / 1024 / 1024, 'MB')     # 1310 MB

z = x[1:] # deep copy
print('x, y, z', process.memory_info().rss / 1024 / 1024, 'MB')  # 1566 MB

Runtime

## Define function in the runtime
# import sys
#
# for line in sys.stdin:
#     line = line.rstrip()
#     if int(line) > 0:
#         def foo():
#             print line, 'positive'
#     else:
#         def foo():
#             print line, 'negative'
#
#     foo()

## Pit falls of closure
# from __future__ import print_function
# def buggy_maker(my_list):
#     acts = []
#     for i in my_list:
#         acts.append(lambda: print(i))
#     return acts
# for act in buggy_maker([1,2,3]):
#     act() # always print 3, instead of 1, 2, 3
#
# def buggy_maker2():
#     acts = []
#     i = 1
#     acts.append(lambda: print(i))
#     i = 2
#     acts.append(lambda: print(i))
#     return acts
# for act in buggy_maker2():
#     act() # always print 2, instead of 1, 2

## Usage of global
# def foo():
#     global s
#     s = None
#     print s
#
# foo()
# print s

Naming

Identifier Type Format Example
Class Camel case class StringManipulator():
Function Words joined by _ def multi_word_name(words):
Constant All uppercase SECRET_KEY = 42

Basically everything not listed should follow the variable/function naming conventions of ‘Words joined by an underscore’.

Control flow

# Avoid repeating variable name in compound if statement
is_generic_name = name in ('Tom', 'Dick', 'Harry')

# Use else to execute code after a for loop concludes
for user in get_all_users():
    print ('Checking {}'.format(user))
    for email_address in user.get_all_email_addresses():
        if email_is_malformed(email_address):
            print ('Has a malformed email address!')
            break
else:
    # The else clause is executed after the iterator is
    # exhausted, unless the loop was ended prematurely
    # due to a break statement
    print ('All email addresses are valid!')

Generator

import random

def get_data():
    """Return 3 random integers between 0 and 9"""
    return random.sample(range(10), 3)

def consume():
    """Displays a running average across lists of integers sent to it"""
    running_sum = 0
    data_items_seen = 0

    while True:
        data = yield
        data_items_seen += len(data)
        running_sum += sum(data)
        print('The running average is {}'.format(running_sum / float(data_items_seen)))

def produce(consumer):
    """Produces a set of values and forwards them to the pre-defined consumer
    function"""
    while True:
        data = get_data()
        print('Produced {}'.format(data))
        consumer.send(data)
        yield

if __name__ == '__main__':
    consumer = consume()
    consumer.send(None)
    producer = produce(consumer)

    for _ in range(10):
        print('Producing...')
        next(producer)

Class

from __future__ import print_function

class Foo(object):
    def __init__(self):
        self.name = 'name'
        self._name = '_name'
        self.__name = '__name'

class Bar(Foo):
    def __init__(self):
        super(Bar, self).__init__()

foo = Foo()
bar = Bar()

print(foo.name)       # name
print(foo._name)      # _name
print(foo.__name)     # ValueError
print(foo._Foo__name) # field __name has been changed to _Foo__name
print(bar.name)       # name
print(bar._name)      # _name
print(bar.__name)     # ValueError
print(foo._Foo__name) # field __name has been changed to _Foo__name

Data Structure

# List
#  Avoid using '', [], and {} as default parameters to functions
def f(a, L=[]):
    L.append(a)
    return L
print(f(1))
print(f(2))
print(f(3))
# This will print
# [1]
# [1, 2]
# [1, 2, 3]


# Dict:
log_severity = None
if 'severity' in configuration:
    log_severity = configuration['severity']
else:
    log_severity = 'Info'
# is equivalent to
log_severity = configuration.get('severity', 'Info')

# Set: the mathematical set operations
A & B # Intersection
A | B # Union
A - B # The set of elements in A but not B, not necessarily equal to B -A
A ^ B # Symmetric Difference

Decorator

Timeit

from __future__ import print_function
from timeit import default_timer as timer

def timeit(F):
    def FF(*args, **kwargs):
        start = timer()
        F(*args, **kwargs)
        end = timer()
        print(F.__name__ + ':', end - start)

    return FF

# This is equivalent to F = timeit(F)
@timeit
def computation_func(x):
    count = 0
    for i in xrange(x):
        count += i

class Foo(object):
    @timeit
    def computation_func(self, x):
        count = 0
        for i in xrange(x):
            count += i

computation_func(2**8)        # 2.78949737549e-05
Foo().computation_func(2**8)  # 2.40802764893e-05

Registry

from __future__ import print_function

registry = {}
def register(obj):
    registry[obj.__name__] = obj
    return obj

@register
def spam(x):
    return (x ** 2)

@register
def ham(x):
    return (x ** 3)

print('Registry:')
for name in registry:
    print(name, '=>', registry[name], type(registry[name]))
# Registry:
# ham => <function ham at 0x7fd0de4b48c0> <type 'function'>
# spam => <function spam at 0x7fd0de4b4848> <type 'function'>

print('\nRegistry calls:')
for name in registry:
    print(name, '=>', registry[name](2)) # Invoke from registry
# Registry calls:
# ham => 8
# spam => 4
## Naming

| Identifier Type | Format            | Example                       |
|-----------------|-------------------|-------------------------------|
| Class           | Camel case        | `class StringManipulator():`  |
| Function        | Words joined by _ | `def multi_word_name(words):` |
| Constant        | All uppercase     | `SECRET_KEY = 42`             |

Basically everything not listed should follow the variable/function naming conventions
of ‘Words joined by an underscore’.

## Control flow

```python
# Avoid repeating variable name in compound if statement
is_generic_name = name in ('Tom', 'Dick', 'Harry')

# Use else to execute code after a for loop concludes
for user in get_all_users():
    print ('Checking {}'.format(user))
    for email_address in user.get_all_email_addresses():
        if email_is_malformed(email_address):
            print ('Has a malformed email address!')
            break
else:
    # The else clause is executed after the iterator is
    # exhausted, unless the loop was ended prematurely
    # due to a break statement
    print ('All email addresses are valid!')

Generator

G = (x ** 2 for x in range(4)) # Generator expression: make an iterable
import random

def get_data():
    """Return 3 random integers between 0 and 9"""
    return random.sample(range(10), 3)

def consume():
    """Displays a running average across lists of integers sent to it"""
    running_sum = 0
    data_items_seen = 0

    while True:
        data = yield
        data_items_seen += len(data)
        running_sum += sum(data)
        print('The running average is {}'.format(running_sum / float(data_items_seen)))

def produce(consumer):
    """Produces a set of values and forwards them to the pre-defined consumer
    function"""
    while True:
        data = get_data()
        print('Produced {}'.format(data))
        consumer.send(data)
        yield

if __name__ == '__main__':
    consumer = consume()
    consumer.send(None)
    producer = produce(consumer)

    for _ in range(10):
        print('Producing...')
        next(producer)

Class

from __future__ import print_function

class Foo(object):
    def __init__(self):
        self.name = 'name'
        self._name = '_name'
        self.__name = '__name'

class Bar(Foo):
    def __init__(self):
        super(Bar, self).__init__()

foo = Foo()
bar = Bar()

print(foo.name)       # name
print(foo._name)      # _name
print(foo.__name)     # ValueError
print(foo._Foo__name) # field __name has been changed to _Foo__name
print(bar.name)       # name
print(bar._name)      # _name
print(bar.__name)     # ValueError
print(foo._Foo__name) # field __name has been changed to _Foo__name

Data Structure

# List
#  Avoid using '', [], and {} as default parameters to functions
def f(a, L=[]):
    L.append(a)
    return L
print(f(1))
print(f(2))
print(f(3))
# This will print
# [1]
# [1, 2]
# [1, 2, 3]


# Dict:
log_severity = None
if 'severity' in configuration:
    log_severity = configuration['severity']
else:
    log_severity = 'Info'
# is equivalent to
log_severity = configuration.get('severity', 'Info')

# Set: the mathematical set operations
A & B # Intersection
A | B # Union
A - B # The set of elements in A but not B, not necessarily equal to B -A
A ^ B # Symmetric Difference

Decorator

Timeit

from __future__ import print_function
from timeit import default_timer as timer

def timeit(F):
    def FF(*args, **kwargs):
        start = timer()
        F(*args, **kwargs)
        end = timer()
        print(F.__name__ + ':', end - start)

    return FF

# This is equivalent to F = timeit(F)
@timeit
def computation_func(x):
    count = 0
    for i in xrange(x):
        count += i

class Foo(object):
    @timeit
    def computation_func(self, x):
        count = 0
        for i in xrange(x):
            count += i

computation_func(2**8)        # 2.78949737549e-05
Foo().computation_func(2**8)  # 2.40802764893e-05

Registry

from __future__ import print_function

registry = {}
def register(obj):
    registry[obj.__name__] = obj
    return obj

@register
def spam(x):
    return (x ** 2)

@register
def ham(x):
    return (x ** 3)

print('Registry:')
for name in registry:
    print(name, '=>', registry[name], type(registry[name]))
# Registry:
# ham => <function ham at 0x7fd0de4b48c0> <type 'function'>
# spam => <function spam at 0x7fd0de4b4848> <type 'function'>

print('\nRegistry calls:')
for name in registry:
    print(name, '=>', registry[name](2)) # Invoke from registry
# Registry calls:
# ham => 8
# spam => 4

22. Modules: The Big Picture

The five major components of the module import search path are

  1. the top-level script’s home directory (the directory containing it)
  2. all directories listed in the PYTHONPATH environment variable
  3. the standard library directories
  4. all directories listed in .pth path files located in standard places
  5. the site-packages root directory for third-party extension installs.

A module will only be imported once in every execution.

# Diamond Dependency
#        +---+
#        |   |
#   +----+ A +----+
#   |    |   |    |
#   |    +---+    |
# +-v-+         +-v-+
# |   |         |   |
# | B |         | C |
# |   |         |   |
# +-+-+         +-+-+
#   |    +---+    |
#   |    |   |    |
#   +----> D <----+
#        |   |
#        +---+
#
# A import B, C
# B import D
# C import D
#
# $: python A.py gives
# - A.py
# - /home/tonyyang/Survey/cpython/experiment/B.pyc
# - /home/tonyyang/Survey/cpython/experiment/D.pyc
# + /home/tonyyang/Survey/cpython/experiment/D.pyc
# + /home/tonyyang/Survey/cpython/experiment/B.pyc
# - /home/tonyyang/Survey/cpython/experiment/C.pyc
# + /home/tonyyang/Survey/cpython/experiment/C.pyc
# + A.py
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment