how to run certain function until we got desired values, python

I have one for loop by which I am generating some random daily value for each month. I need to make it sure that the generated monthly mean value should equal to the observed mean value (provided in different dataframe). So I need to run this for loop until the desired value is generated and after that it again run for next month.

I have sample code but not sure how to run it several time until to get desired value:

for j in np.arange(1,13,1):
    # calculating monthly leanth from provided datasets 
    mlen = ((df_test[df_test.index.year==2000]).index.month==j).sum()

    # fit to my distribution
    phat = stats.exponweib.fit((df_test[df_test.index.month==j].mean(axis=1)).dropna())

    # predicting based on the fitted distribution
    esti = stats.exponweib.rvs(*phat,int(mlen))

    if esti.mean() == monthly_mean[0]

Where monthly_mean is the observed monthly mean value is its look like below, where the first value for the first month.

monthly_mean = array([  5.15    ,   5.948571,   7.028261,   4.144231,   3.585965,
         5.244828,   4.915455,   4.757   ,   5.803614,  12.573684,
         5.683333,   3.875   ])

any help/suggestion is highly appriciated!