How to loop for choosing data in Jupyter Notebook by Python - python-3.x

Question

I have 2 columns in data frames.
Column 1 = Source
Column 2 = Target
My data source
Source Target
A B
W X
B C
C D
C Z
A Z
Z Y
Input = A, The output should be display as below.
Source Target
A B
B C
C D
C Z
A Z
Z Y
I try to code as below but not finished yet.
In [1]:
a = input()
b = []
for Source, Target in zip(data.Source,data.Target):
if Source == a:
b.append(True)
else:
b.append(False)
Input = A
In [2]: is_long = pd.Series(b)
is_long
Out [2]: 0 True
1 False
2 True
3 True
4 ...
In [3]: data[is_long]
Out [3]: Source Target
A B
B C
C D
C Z
A Z
Z Y

Answer 1

As I understood, the idea is:
try in turn each vertice from the source DataFrame,
the current vertice is "OK" when its Source node has been visited
before,
the visited list contains initially only the node given by the user and
should be extended by Target node of each checked vertice.
Start from defining the following class:
class Visitor:
def __init__(self):
self.clear()
def addNode(self, node):
if not self.isVisited(node):
self._nodes.append(node)
def isVisited(self, node):
return node in self._nodes
def clear(self):
self._nodes = []
This class keeps a register of visited nodes. It will be used soon.
Then define a function checking the "continuation criterion" for the current
row:
def isContinued(row, vs):
res = vs.isVisited(row.Source)
if res:
vs.addNode(row.Target)
return res
The first argument is the current row and the second is a Visitor object.
Then run:
vs = Visitor()
vs.addNode(a)
df[df.apply(isContinued, axis=1, vs=vs)]
The first line creates a Visitor object.
The second adds the "starting node" (just given by the user) to the
"visited list".
Then df.apply(isContinued, axis=1, vs=vs) creates a Boolean vector
- the continuation criteria for edges.
As isContinued function is applied to subsequent edges, the "visited list"
is extended with Target node.
The (just updated) visited list is then used to compute the continuation
criteria for subsequent edges.
The result is a list of edges meeting the continuation criterion.