遍历 python 字典并从列表中返回匹配值

Question

I am having a hard time to get this code running and hope to get help from everyone. I have a dictionary and a list that i need to iterate through the list and return the dictionary with matching value. Here is the code I have so far and the desire output that i can print out to excel as well

allJobs = {'A':[1,2,3],
           'B':[2,3,4],
           'C':[1,3],
           'D':[3,4]}

Jobs = ['A','C']

JobsWithNums = {}
matchJobs =[]


for jobs in allJobs:
    if jobs in Jobs:
        JobsWithNums.append(jobs)
        
print(JobsWithNums)
JobsWithNums = {'A':[1,2,3],
                'C':[1,3]}

Answer 1

Cause of Error -

Dictionary in python has no append() method and hence you would be getting an error something like this -

<module>AttributeError: 'dict' object has no attribute 'append'>

So, the append method does not work with dictionary for adding new key:value pairs. So you have to use it the way I have shown in the code below.

---

Correct Solution -

You have to iterate over the keys of dictionary and append the value of key when you find the matching key. You should be doing it as follows -

allJobs = {'A':[1,2,3],
           'B':[2,3,4],
           'C':[1,3],
           'D':[3,4]}

Jobs = ['A','C']

JobsWithNums = {}
matchJobs =[]


for jobs in allJobs.keys():
    if jobs in Jobs:
        JobsWithNums[jobs] = allJobs[jobs]
      # ^ : Adding the value corresponding to jobs when jobs is present in Jobs list
      # You were trying to do with append which is not the valid way to add a key to dictionary
print(JobsWithNums)

Output:

{'A': [1, 2, 3], 'C': [1, 3]}

Do note that the append method does not work with dictionary. So you have to use it the way I have done it in code. You could also do it usingdict.update() method. Refer the following snippet -

for jobs in allJobs.keys():
    if jobs in Jobs:
        JobsWithNums.update({jobs:allJobs[jobs]})
       # Another possible variant =>
       # JobsWithNums.update(jobs=allJobs[jobs])

But this would probably be an inefficient way to do it as compared to the first one. Use it only if you have to update multiple keys into dictionary at once.

---

Alternate way of doing it -

A shorter (pythonic) way of doing it would be as follows =>

allJobs = {'A':[1,2,3],'B':[2,3,4],'C':[1,3],'D':[3,4]}    
Jobs = ['A','C']
               # The below code just says, add job_name:value pair if job_name exists in Jobs list
JobsWithNums = {job_name: value for job_name, value in allJobs.items()
                if job_name in Jobs}

print(JobsWithNums)

Output:

{'A': [1, 2, 3], 'C': [1, 3]}

You can learn more about these here - python docs

Hope this helps !

Answer 2

You can try this:

for job in allJobs.keys():
    if job in Jobs:
        JobsWithNums[job] = alljobs[job]

The.key() here returns the names of the jobs (A, B, C, D) and you later reference it like alljobs to get the value.

Answer 3

The append method does not work the way you expect with dictionaries. You can do the following in your loop to get around this:

for jobs, nums in allJobs.items():
    if jobs in Jobs:
        JobsWithNums[jobs] = nums

This is iterating over all of the keys and values in your dictionary and creating those entries in the new JobsWithNums dict.

Alternatively, you can build the same list in one line using dict comprehension:

JobsWithNums = {k:v for k,v in allJobs if k in Jobs}

Answer 4

You can do this fairly easily with a dictionary comprehension:

jobsWithNums = {job_name: value for job_name, value in allJobs.items()
                if job_name in Jobs}
print(jobsWithNums)

And the output is:

{'A': [1, 2, 3], 'C': [1, 3]}

Answer 5

A better way to do this is iterating through the list and if that element exists in the dictionary append it to JobsWithNums. An advantage of this is you only need to iterate through the list once and the lookup time for the dictionary is O(1) so it will be more efficient.

JobsWithNums = {}
for job in Jobs:
    if allJobs.get(job):
        JobsWithNums[job] = allJobs[job]

get() on dictionary return None if key is not present

Answer 6

Thank you for all the helps and I have been able to run it. Also I am trying to export the result to excel using pandas data frame but not being able to do so and run into the error of arrays must all e same length - and how can i name the columns of the data frame as Jobs, Nums

df = pd.DataFrame(JobsWithNums)
df.to_excel("file.xlsx")