I need help with refactoring my program. I have a function that I am calling in two programs. In one program i just need the overtimeAmount, and in the other program I need all three variables returned. How can I refactor this two only return the variables i need for the program.
def OT2(hours, rate):
if hours > 160.0:
overtimeHours = hours - 160.0
overtimeRate = rate * 1.05
overtimeAmount = overtimeHours * overtimeRate
else:
overtimeAmount = 0.0
overtimeRate = 0.0
overtimeHours = 0.0
return overtimeHours, overtimeRate, overtimeAmount
Instead of having one function that does two things, use two functions. That'll go a long way in terms of maintainability and follows the Unix philosophy of doing one thing well:
def get_overtime_hours(hours):
return max(0, hours - 160)
def get_overtime_info(hours, rate):
overtime_hours = get_overtime_hours(hours)
if overtime_hours > 0:
overtime_rate = rate * 1.05
overtime_amount = overtime_hours * overtime_rate
else:
overtime_amount = 0.0
overtime_rate = 0.0
return overtime_hours, overtime_rate, overtime_amount
It's good advice to never return a variable amount of results because it makes your function unpredictable; instead, either
always return the same number, (potentially making unused values None) and let the caller pick what to keep
return a dictionary (one result)
PEP 8 has some words about this in Be consistent in return statements, but doesn't explicitly cover your case
Be consistent in return statements. Either all return statements in a function should return an expression, or none of them should. If any return statement returns an expression, any return statements where no value is returned should explicitly state this as return None, and an explicit return statement should be present at the end of the function (if reachable) [..]
One possibility is to add a third parameter to indicate what you want returned and then an if statement to decide what to return. That doesn't seem any cleaner than just returning all 3 values and ignoring the ones you don't want.
Related
This question already has answers here:
Static variable in Python?
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Closed 1 year ago.
I'm trying to write a function that updates its local variable each time it is run but it is not working for some reason.
def max_equity(max_equity=0):
if current_equity() > max_equity:
max_equity = current_equity()
print(max_equity)
return max_equity
else:
print(max_equity)
return max_equity
and the function which it is calling
def current_equity():
for n in range(len(trade_ID_tracker)-1):
equity_container = 0
if (trade_ID_tracker[n,2]) == 0:
break
else:
if (trade_ID_tracker[n, 1].astype(int) == long):
equity_container += (df.loc[tbar_count,'Ask_Price'] - trade_ID_tracker[n, 2]) * trade_lots * pip_value * 1000
elif (trade_ID_tracker[n, 1].astype(int) == short):
equity_container += 0 - (df.loc[tbar_count,'Ask_Price'] - trade_ID_tracker[n, 2]) * trade_lots * pip_value * 10000
return (current_balance + equity_container)
but for some reason the max_equity() function prints current_equity() which I can only imagine means that either:
if current_equity() > max_equity:
is not doing it's job and is triggering falsely
or
max_equity = current_equity()
is not doing its job and max_equity starts at zero every time it is run.
In other words if I put max_equity() in a loop where current_equity() is
[1,2,3,4,5,4,3,2,1]
then max_equity() should return
[1,2,3,4,5,5,5,5,5]
But instead it returns
[1,2,3,4,5,4,3,2,1]
Here's a quick example test
ar = [1,2,3,4,5,4,3,2,1]
def stuff(max_equity=0):
if ar[n] > max_equity:
max_equity = ar[n]
print(max_equity)
else:
print(max_equity)
for n in range(len(ar)):
stuff()
Either way I'm kind of stumped.
Any advice?
local function variables are reset at each function call. This is essential for the behavior of functions as idempotent, and is a major factor in the success of the procedural programming approach: a function can be called form multiple contexts, and even in parallel, in concurrent threads, and it will yield the same result.
A big exception, and most often regarded as one of the bigger beginner traps of Python is that, as parameters are reset to the default values specified in the function definition for each call, if these values are mutable objects, each new call will see the same object, as it has been modified by previous calls.
This means it could be done on purpose by, instead of setting your default value as 0 you would set it as a list which first element was a 0. At each run, you could update that value, and this change would be visible in subsequent calls.
This approach would work, but it is not "nice" to depend on a side-effect of the language in this way. The official (and nice) way to keep state across multiple calls in Python is to use objects rather than functions.
Objects can have attributes tied to them, which are both visible and writable by its methods - which otherwise have their own local variables which are re-started at each call:
class MaxEquity:
def __init__(self):
self.value = 0
def update(max_equity=0):
current = current_equity()
if current > self.value:
self.value = current
return self.value
# the remainder of the code should simply create a single instance
# of that like ]
max_equity = MaxEquity()
# and eeach time yoiu want the max value, you should call its "update"
# method
I am working at a Python project, and I reached a point where I need a function to stop and return after a x time, that is passed as a parameter. A simple example:
def timedfunc(time_to_stop):
result = None
while (time_has_not_passed):
do()
return result
I explain:
When time has passed, timedfunc stops and interrupts everything in it, and jumps right to return result. So, what I need is a way to make this function work as long as possible (time_to_stop), and then to return the result variable, which is as accurate as possible (More time, more calculations, more accuracy). Of course, when time is out, also do() stops. To better understand, I say that the function is continuosly changing the value of result, and once the time has passed it returns the current value. (do() stands for all the calculations that change result)
I just made a simple example to better explain what I want:
def multiply(time):
result = 10
while time_has_not_passed:
temporary = result*10 #Actually much more time-consuming, also like 3 minutes.
temporary /= 11
result = temporary
return result
This explains what kind of calculations do() makes, and I need as many *10/11 as python can do in, for example, 0.5 sec.
I know that this pretty complicated, but any help would be great.
import time
start_time = time.time()
program()
if (time.time()-start_time)>0.5: #you can change 0.5 to any other value you want
exit()
It is something like this. you can put this if statement right inside your program function too.
maybe you can use:
time.sleep(x)
do()
# OR
now = time()
cooldown = x
if now + cooldown < time():
do()
if you want it to do something for a while
now = time()
needed_time = x
while now + needed_time > time():
do()
Still fairly new to python.
I was wondering what would be a good way of detecting what output response a python program were to choose.
As an example, if you were to make a speed/distance/time calculator, if only 2 input were ever given, how would you detect which was the missing input and therefore the output? I can think of some fairly crude ways but I was wondering if there was anything else if more complex tasks were to come into play.
I guess something like:
def sdf(speed=0, distance=0, time=0):
# detect which parameter has no input / equals 0
# calculate result
# return result
sdf(speed=10, distance=2)
Any ideas?
Python allows you to change types of variables on the fly. Since you are working with integers and 0 could be a useful value in your calculations, your default 'not present' value should be None:
def sdf(speed=None, time=None, distance=None):
if speed is None:
return calculate_speed(time, distance), time, distance
if time is None:
return speed, calculate_time(speed, distance), distance
if distance is None:
return speed, time, calculate_distance(speed, time)
# All paramters have been set! Maybe check if all three are correct
return speed, time, distance
speed, time, distance = sdf(speed=1, distance=2)
This way you don't have to find out what happened afterwards. This function will give you all three values, given you gave it at least 2 out of the 3.
If your program flow allows for multiple values be None, your functions calculate_XY should throw an exception if they detect it. So in this case:
def calculate_distance(speed, time)
return speed * time
It will throw an unsupported operand exception(TypeError), so no need to clutter your code with useless asserts.
If you really don't know how many parameters will be set, do something like this:
try:
retval = sdf(None, None, x)
except TypeError as e:
print(e)
handle_exception(e)
Also just a heads up: the is operator in Python checks if the objects are the same object, not their value. Since objects that are assigned to None are just a 'pointer to the global None object'(simplification), checking whether a value 'contains' None with is is preferred. However be aware of this:
a = b = list()
a is b
True
# a and b are 'pointers' to the same list object
a = list()
b = list()
a is b
False
a == b
True
# a and b contain 2 different list objects, but their contents are identical
Just be aware that to compare values use == and to check if they are the same object, use is.
HTH
This is what I would do :
def sdf(distance=None, speed=None, time=None):
"""Calculate the missing speed, distance time value
returns a 3-tuple (speed, distance, time)
raises ValueError if more than one or no unknowns are given"""
if (distance, speed,time).count(None) > 1:
raise ValueError('Error - more than one unknown provided')
if (distance, speed,time).count(None) == 0:
raise ValueError('Not sure what to calculate - all paramaters provided')
if speed is None:
return distance/time, distance, time
if time is None:
return speed, distance, distance/speed
if distance is None:
return speed, speed*time, time
You should use multiple functions and call the one needed.
def CalculateTravelTime(distance, speed)
def CalculateTravelSpeed(distance, time)
def CalculateTravelDistance(speed, time)
I use global variables but I've read that they aren't a good practice or pythonic. I often use functions that give as a result many yes/no variables that I need to use in the main function. For example, how can I write the following code without using global variables?
def secondary_function():
global alfa_is_higher_than_12
global beta_is_higher_than_12
alfa = 12
beta = 5
if alfa > 10:
alfa_is_higher_than_12 = "yes"
else:
alfa_is_higher_than_12 = "no"
if beta > 10:
beta_is_higher_than_12 = "yes"
else:
beta_is_higher_than_12 = "no"
def main_function():
global alfa_is_higher_than_12
global beta_is_higher_than_12
secondary_function()
if alfa_is_higher_than_12=="yes":
print("alfa is higher than 12")
else:
print("alfa isn't higher than 12")
if beta_is_higher_than_12=="yes":
print("beta is higher than 12")
else:
print("beta isn't higher thant 12")
main_function()
The term "Pythonic" doesn't apply to this topic--using globals like this is poor practice in any programming language and paradigm and isn't something specific to Python.
The global keyword is the tool that Python provides for you to opt out of encapsulation and break the natural scope of a variable. Encapsulation means that each of your components is a logical, self-contained unit that should work as a black box and performs one thing (note: this one thing is conceptual and may consist of many, possibly non-trivial, sub-steps) without mutating global state or producing side effects. The reason is modularity: if something goes wrong in a program (and it will), having strong encapsulation makes it very easy to determine where the failing component is.
Encapsulsation makes code easier to refactor, maintain and expand upon. If you need a component to behave differently, it should be easy to remove it or adjust it without these modifications causing a domino effect of changes across other components in the system.
Basic tools for enforcing encapsulation include classes, functions, parameters and the return keyword. Languages often provide modules, namespaces and closures to similar effect, but the end goal is always to limit scope and allow the programmer to create loosely-coupled abstractions.
Functions take in input through parameters and produce output through return values. You can assign the return value to variables in the calling scope. You can think of parameters as "knobs" that adjust the function's behavior. Inside the function, variables are just temporary storage used by the function needed to generate its one return value then disappear.
Ideally, functions are written to be pure and idempotent; that is, they don't modify global state and produce the same result when called multiple times. Python is a little less strict about this than other languages and it's natural to use certain in-place functions like sort and random.shuffle. These are exceptions that prove the rule (and if you know a bit about sorting and shuffling, they make sense in these contexts due to the algorithms used and the need for efficiency).
An in-place algorithm is impure and non-idempotent, but if the state that it modifies is limited to its parameter(s) and its documentation and return value (usually None) support this, the behavior is predictable and comprehensible.
So what does all this look like in code? Unfortunately, your example seems contrived and unclear as to its purpose/goal, so there's no direct way to transform it that makes the advantages of encapsulation obvious.
Here's a list of some of the problems in these functions beyond modifying global state:
using "yes" and "no" string literals instead of True/False boolean values.
hardcoding values in functions, making them entirely single-purpose (they may as well be inlined).
printing in functions (see side effects remark above--prefer to return values and let the calling scope print if they desire to do so).
generic variable names like secondary_function (I'm assuming this is equivalent to foo/bar for the example, but it still doesn't justify their reason for existence, making it difficult to modify as a pedagogical example).
But here's my shot anyway:
if __name__ == "__main__":
alpha = 42
beta = 6
print("alpha %s higher than 12" % ("is" if alpha > 12 else "isn't"))
print("beta %s higher than 12" % ("is" if beta > 12 else "isn't"))
We can see there's no need for all of the functions--just write alpha > 12 wherever you need to make a comparison and call print when you need to print. One drawback of functions is that they can serve to hide important logic, so if their names and "contract" (defined by the name, docstring and parameters/return value) aren't clear, they'll only serve to confuse the client of the function (yourself, generally).
For sake of illustration, say you're calling this formatter often. Then, there's reason to abstract; the calling code would become cumbersome and repetitive. You can move the formatting code to a helper function and pass any dynamic data to inject into the template:
def fmt_higher(name, n, cutoff=12):
verb = "is" if n > cutoff else "isn't"
return f"{name} {verb} higher than {cutoff}"
if __name__ == "__main__":
print(fmt_higher("alpha", 42))
print(fmt_higher("beta", 6))
print(fmt_higher("epsilon", 0))
print(fmt_higher(name="delta", n=2, cutoff=-5))
We can go a step further and pretend that n > cutoff was a much more complicated test with many small steps that would breach single-responsibility if left in fmt_higher. Maybe the complicated test is used elsewhere in the code and could be generalized to support both use cases.
In this situation, you can still use parameters and return values instead of global and perform the same sort of abstraction to the predicate as you did with the formatter:
def complex_predicate(n, cutoff):
# pretend this function is much more
# complex and/or used in many places...
return n > cutoff
def fmt_higher(name, n, cutoff=12):
verb = "is" if complex_predicate(n, cutoff) else "isn't"
return f"{name} {verb} higher than {cutoff}"
if __name__ == "__main__":
print(fmt_higher("alpha", 42))
print(fmt_higher("beta", 6))
print(fmt_higher("epsilon", 0))
print(fmt_higher(name="delta", n=2, cutoff=-5))
Only abstract when there is sufficient reason to abstract (the calling code becomes clogged or when you're repeating similar blocks of code multiple times are classic rules-of-thumb). And when you do abstract, do it properly.
One could ask what reasons you might have to structure your code like this, but assuming you have your reasons, you could just return the values from your secondary function:
def secondary_function():
alfa = 12
beta = 5
if alfa > 10:
alfa_is_higher_than_12 = "yes"
else:
alfa_is_higher_than_12 = "no"
if beta > 10:
beta_is_higher_than_12 = "yes"
else:
beta_is_higher_than_12 = "no"
return alfa_is_higher_than_12, beta_is_higher_than_12
def main_function():
alfa_is_higher_than_12, beta_is_higher_than_12 = secondary_function()
if alfa_is_higher_than_12=="yes":
print("alfa is higher than 12")
else:
print("alfa isn't higher than 12")
if beta_is_higher_than_12=="yes":
print("beta is higher than 12")
else:
print("beta isn't higher thant 12")
Never write 'global'. Then you are sure you are not introducing any global variables.
You could also pass the values as arguments:
def secondary_function():
alfa = 12
beta = 5
if alfa > 10:
alfa_is_higher_than_12 = "yes"
else:
alfa_is_higher_than_12 = "no"
if beta > 10:
beta_is_higher_than_12 = "yes"
else:
beta_is_higher_than_12 = "no"
return alfa_is_higher_than_12, beta_is_higher_than_12
def main_function(alfa_is_higher_than_12, beta_is_higher_than_12):
if alfa_is_higher_than_12=="yes":
print("alfa is higher than 12")
else:
print("alfa isn't higher than 12")
if beta_is_higher_than_12=="yes":
print("beta is higher than 12")
else:
print("beta isn't higher thant 12")
main_function(*secondary_function())
I am quering a database for some paramaters which depend on a attribute called count! count can be incremented incase the 1st query does not return anything. Here is a sample code
sls = {(213.243, 55.556): {}, (217.193, 55.793): {}, (213.403, 55.369): {}}
for key in sls.keys:
if not sls[key]:
ra, dec = key[0], key[1]
search_from_sourcelist(sl, ra,dec)
count = 1
def search_from_sourcelist(sl, ra,dec):
dist = count/3600.0
sls[(ra,dec)] = sl.sources.area_search(Area=(ra,dec,dist))
return
Incase i run the method search_from_sourcelist, and it doesnt return anything, i would like to increment count, and do the query again. This is to be done for all keys in sls dictionary, untill all the keys have a value!!
Here is the most fundamental recursive function
def countdown(n):
if n == 0:
return "Blastoff"
else:
print "T minus %s" % n
return countdown(n-1)
You will notice that countdown returns itself with a modified argument, in this case n but -1, so if you actually followed this all the way through you would get
(-> indicates a call)
countdown(5) -> countdown(4) -> countdown(3) -> countdown(2) -> countdown(1) -> countdown(0) #stop
So now you understand what a recursive function looks like you realize you never actually return a call of your own function, thus your code is not recursive
We use recursion because we want to boil a task down to its simplest form then work from there, so a good example of this would be the mcnuggets problem. So you need to tell us what you are trying to achieve and how it can be made a smaller problem (or more importantly why.) Are you sure you cannot do this iteratively? remember you don't want to blow your stack depth because python is NOT tail recursive by standard
Recursion is useful when you find a way to reduce the initial problem to a "smaller version of itself".
The standard example is the factorial function
def fac(n):
return n * fac(n-1) if n > 1 else 1
Here you reduce the problem of calculating the factorial of n to calculating the factorial of n-1.
In your code there is no such "reduction". You just increment a value and start the same problem over again. Thus, I recommend you solve it iteratively.
I'm not sure that you need a recursive algorithm for this.
Incase i run the method search_from_sourcelist, and it doesnt return anything, i would like to increment count, and do the query again. This can be done with a while loop as follows:
for key, value in sls.iteritems():
if not value:
ra, dec = key[0], key[1]
count = 1
while not search_from_sourcelist(sls, ra, dec):
count += 1
But if you really do want to do this recursively, you can do it as follows, leave a comment and I'll write it up.
Further, you should look into your search_from_sourcelist function, as it always returns None