Python odds and ends scope, filter, reduce
| description | code | comments | |
| quickly assign values | a,b,c = (3,7,12) | unpack | |
| nested functions | outer func return inner func | ||
Python's built-in scope¶
- check out Python's built-in scope, which is really just a built-in module called builtins
- to query builtins, you'll need to import builtins
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import builtins
print dir(builtins)
nested functions¶
- return innter function
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def raise_val(n):
"""Return the inner function."""
def inner(x):
"""Raise x to the power of n."""
raised = x ** n
return raised
return inner
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square = raise_val(2)
cube = raise_val(3)
print square(6), cube(6)
pass parameters¶
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raise_val(4)(3)
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scope searched¶
- Local scope
- Enclosing functions
- Global
- Built-in
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n=3
def outer():
"""Prints the value of n."""
n = 1
def inner():
n = 2
print(n)
inner()
print(n)
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outer()
nested func¶
- nesting functions is the idea of a closure
- This means that the nested or inner function remembers the state of its enclosing scope when called
- Thus, anything defined locally in the enclosing scope is available to the inner function even when the outer function has finished execution
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# Define echo
def echo(n):
"""Return the inner_echo function."""
# Define inner_echo
def inner_echo(word1):
"""Concatenate n copies of word1."""
echo_word = word1 * n
return echo_word
# Return inner_echo
return inner_echo
# Call echo: twice
twice = echo(2)
# Call echo: thrice
thrice = echo(3)
# Call twice() and thrice() then print
print(twice('hello'), thrice('hello'))
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echo(7)('wtf ')
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# Define gibberish
def gibberish(*notmatter):
"""Concatenate strings in *args together."""
# Initialize an empty string: hodgepodge
hodgepodge = ''
# Concatenate the strings in args
for word in notmatter:
hodgepodge += word+ ' '
# Return hodgepodge
return hodgepodge
# Call gibberish() with one string: one_word
one_word = gibberish("luke")
# Call gibberish() with five strings: many_words
many_words = gibberish("luke", "leia", "han", "obi", "darth")
# Print one_word and many_words
print(one_word)
print(many_words)
Function with variable-length keyword arguments (**kwargs)¶
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# Define report_status
def report_status(**whatevername):
"""Print out the status of a movie character."""
print("\nBEGIN: REPORT\n")
print whatevername
print
# Print a formatted status report
for key, value in whatevername.items():
print(key + ": " + value)
print("\nEND REPORT")
# First call to report_status()
report_status(name="luke", affiliation="jedi", status="missing")
# Second call to report_status()
report_status(name="anakin", affiliation="sith lord", status="deceased")
Map() and lambda functions¶
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# Create a list of strings: spells
spells = ['protego', 'accio', 'expecto patronum', 'legilimens']
# Use map() to apply a lambda function over spells: shout_spells
shout_spells = map(lambda item: item + '!!!', spells)
# Convert shout_spells to a list: shout_spells_list
shout_spells_list = list(shout_spells)
# Convert shout_spells into a list and print it
print(shout_spells_list)
Filter() and lambda functions¶
The function filter() offers a way to filter out elements from a list that doesn't satisfy certain criteria.¶
filter(function or None, sequence) -> list, tuple, or string
Return those items of sequence for which function(item) is true. If function is None, return the items that are true. If sequence is a tuple or string, return the same type, else return a list.
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# Create a list of strings: fellowship
fellowship = ['frodo', 'samwise', 'merry', 'aragorn', 'legolas', 'boromir', 'gimli']
# Use filter() to apply a lambda function over fellowship: result
result = filter(lambda member: len(member) > 6, fellowship)
# Convert result to a list: result_list
result_list = list(result)
# Convert result into a list and print it
print(result_list)
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filter(lambda member: len(member) >3, ['1234','234','34567'])
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filter(None, [12>1, 'wtf' if 2>1 else 0, 'aiya' if 3>2 else 7, 'momomo' if 4>5 else -44])
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Reduce() and lambda functions¶
The reduce() function is useful for performing some computation on a list and, unlike map() and filter(), returns a single value as a result.¶
To use reduce(), you must import it from the functools module.¶
- reduce(function, sequence[, initial]) -> value
- Apply a function of two arguments cumulatively to the items of a sequence, from left to right, so as to reduce the sequence to a single value.
- For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5).
- If initial is present, it is placed before the items of the sequence in the calculation, and serves as a default when the sequence is empty.
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# Import reduce from functools
from functools import reduce
# Create a list of strings: stark
stark = ['robb', 'sansa', 'arya', 'eddard', 'jon']
# Use result() to apply a lambda function over stark: result
result = reduce(lambda item1, item2: item1 +' '+ item2, stark)
# Print the result
print(result)
error handling¶
- raise error
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try: '3'+3
except Exception, e: print e
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# Define shout_echo
def shout_echo(word1, echo=1):
"""Concatenate echo copies of word1 and three
exclamation marks at the end of the string."""
# Raise an error with raise
if echo < 0:
raise ValueError('echo must be greater than 0')
# Concatenate echo copies of word1 using *: echo_word
echo_word = word1 * echo
# Concatenate '!!!' to echo_word: shout_word
shout_word = echo_word + '!!!'
# Return shout_word
return shout_word
# Call shout_echo
try:
shout_echo("particle", echo=-3)
except Exception, e:
print e
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shout_echo("123", echo=-1)
use filter(lambda: x ...) in Pandas¶
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# Select retweets from the Twitter dataframe: result
result = filter(lambda x: x[0:2] == 'RT', df['text'])
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