[英]Python perform operation in string
因此,我試圖將變量操作(用戶定義的)傳遞給函數,並且在嘗試找到一種好的方法方面遇到麻煩。 我能想到的就是將所有選項硬編碼到函數中,如下所示:
def DoThings(Conditions):
import re
import pandas as pd
d = {'time' : pd.Series([1., 2., 3., 4.], index=['a', 'b', 'c', 'd']),
'legnth' : pd.Series([4., 5., 6., 7.], index=['a', 'b', 'c', 'd'])}
df = pd.DataFrame(d)
print df
for Condition in Conditions:
# Split the condition into two parts
SplitCondition = re.split('<=|>=|!=|<|>|=',Condition)
# If the right side of the conditional statement is a number convert it to a float
if SplitCondition[1].isdigit():
SplitCondition[1] = float(SplitCondition[1])
# Perform the condition specified
if "<=" in Condition:
df = df[df[SplitCondition[0]]<=SplitCondition[1]]
print "one"
elif ">=" in Condition:
df = df[df[SplitCondition[0]]>=SplitCondition[1]]
print "two"
elif "!=" in Condition:
df = df[df[SplitCondition[0]]!=SplitCondition[1]]
print "three"
elif "<" in Condition:
df = df[df[SplitCondition[0]]<=SplitCondition[1]]
print "four"
elif ">" in Condition:
df = df[df[SplitCondition[0]]>=SplitCondition[1]]
print "five"
elif "=" in Condition:
df = df[df[SplitCondition[0]]==SplitCondition[1]]
print "six"
return df
# Specify the conditions
Conditions = ["time>2","legnth<=6"]
df = DoThings(Conditions) # Call the function
print df
結果是:
legnth time
a 4 1
b 5 2
c 6 3
d 7 4
five
one
legnth time
c 6 3
一切都很好,但是我想知道是否存在一種更好或更有效的方法,可以將條件傳遞到函數中而不用寫出所有if語句。 有任何想法嗎?
解:
def DoThings(Conditions):
import re
import pandas as pd
d = {'time' : pd.Series([1., 2., 3., 4.], index=['a', 'b', 'c', 'd']),
'legnth' : pd.Series([4., 5., 6., 7.], index=['a', 'b', 'c', 'd'])}
df = pd.DataFrame(d)
print df
for Condition in Conditions:
# Split the condition into two parts
SplitCondition = re.split('<=|>=|!=|<|>|=',Condition)
# If the right side of the conditional statement is a number convert it to a float
if SplitCondition[1].isdigit():
SplitCondition[1] = float(SplitCondition[1])
import operator
ops = {'<=': operator.le, '>=': operator.ge, '!=': operator.ne, '<': operator.lt, '>': operator.gt, '=': operator.eq}
cond = re.findall(r'<=|>=|!=|<|>|=', Condition)
df = df[ops[cond[0]](df[SplitCondition[0]],SplitCondition[1])]
return df
# Specify the conditions
Conditions = ["time>2","legnth<=6"]
df = DoThings(Conditions) # Call the function
print df
輸出:
legnth time
a 4 1
b 5 2
c 6 3
d 7 4
legnth time
c 6 3
您可以通過operator
模塊訪問內置運算operator
,然后構建一個表,將您的運算符名稱映射到內置運算符,如下面的示例所示:
import operator
ops = {'<=': operator.le, '>=': operator.ge}
In [3]: ops['>='](2, 1)
Out[3]: True
您可以使用屏蔽做這種操作的(你會發現它速度快了很多 ):
In [21]: df[(df.legnth <= 6) & (df.time > 2)]
Out[21]:
legnth time
c 6 3
In [22]: df[(df.legnth <= 6) & (df.time >= 2)]
Out[22]:
legnth time
b 5 2
c 6 3
注意:由於您的查詢中不應包含b,因此您的實現中存在一個錯誤。
您還可以執行或(使用|
)操作,該操作將按您期望的那樣工作:
In [23]: df[(df.legnth == 4) | (df.time == 4)]
Out[23]:
legnth time
a 4 1
d 7 4
在pandas==0.13
(不確定何時發布…… 0.12
),您將能夠執行以下操作,所有這些操作都是等效的:
res = df.query('(legnth == 4) | (time == 4)')
res = df.query('legnth == 4 | time == 4')
res = df.query('legnth == 4 or time == 4')
和我個人的最愛
res = df['legnth == 4 or time == 4']
query
和__getitem__
都接受一個任意的布爾表達式,並自動在表達式中的每個變量名稱上“前綴”調用框架實例(也可以使用局部變量和全局變量)。 這使您可以:1)比鍵入df.
簡潔得多地表達查詢df.
面對一切2)使用語法來表達查詢,讓我們面對現實,它看起來比丑陋的按位運算符更好; 3)如果您有巨大的框架和非常復雜的表達式,則可能比“純” Python等效要快得多,最后是4 )可讓您將同一查詢傳遞給多個具有共同的列子集的幀(畢竟,它是一個字符串)。
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